In my writing class, huge emphasis was placed on outlining the entire plot. Yet as I write it, things change dramatically, and I end up not following the outline. Is it bad to just go with the flow?
That's understandable. There's always that tempting desire to write things as 'efficiently' as possible, where the time spent planning a particular section turns immediately into results when writing that section.
It's also something that rarely works the way you'd hope.
There are great benefits to knowing where you're going before you write it. There's the obvious stuff: you can hardly foreshadow a key event if you haven't yet already planned that it should happen. But there's always some difficulty in writing down a section if you don't know where it's going, and there's plenty of danger that you'll write yourself into a dead end. And even with the question of the plot changing, it's very reasonable that understanding how the plot needs to change is easier when you know how the plot is supposed to fit together beforehand. (I've spent half a year on my project going nowhere because I actually needed to write another event first to make everything work: better planning could have saved me from that.)
As for the reasoning behind your Creative Writing course setting this... for a course of study like this they are going to want to train everyone to give you an entire toolkit as a writer. I would hardly be surprised if they have exercises where they just give you a prompt and expect an instantly written short story (where you and your fellow pantsers would have an easier time and the planners among your group would be asking stackexchange questions). Your situation is just the flipside of this: your writing course setting the most extreme possible planning task. That way, everyone gets gets the entire panster-planner range of tools and therefore the most broad-ranging possible ways to write. When you become a professional writer, you'll still be a natural at going with the flow but you'll be better at digging yourself out of plotholes than you are now thanks to these exercises.
Depending on what kind of writing you do, you don't necessarily need an outline. If you're writing a short story of few thousand words, then, sure you can just bang it out.
But writing long form fiction, the outline is useful so you know where the story will be going and most importantly how to get there. A big problem is that people often know how to start their story, and know how they want to end, but don't know how to connect those with the story. The outline helps with that by making you think and plan the steps between A and Z.
Now, the plan may change, perhaps you find a better route to your destination. Perhaps you find a better goal. That is ok, the outline is a guide, not a mandate set in stone. It should be a living document. Revise it as needed.
Also, different people work differently, so make your outline as suits you. For some it can be very vague wish list of few story points. For others it can be meticulously planned blueprint that covers absolutely everything in the story. Or anything in between. There is no one set way Thou Must Have An Outline Like This. If you work better with a loose ideas outline, then you work better like that. You don't have to change, unless you want to. The outline is there to help you, not hinder you.
Outlining is just a tool in your writers' toolbox. Many very successful writers never outline at all. But it can be helpful to give your work structure and overall coherence. It's easier to see those big overall patterns from the bird's eye view.
Ironically an outline is one of those things that may be most helpful when you don't stick to it. Think of it as a scaffold that you build your story on. Once the story is strong enough to stand on its own, you can remove the scaffold entirely.
As to why they emphasized it so much in your class --a writing class needs to teach something. It's difficult to teach inspiration, so often writing classes will focus on practical, learnable skills, such as outlining. If it works for you, do it, if it doesn't, discard it. Ultimately the only justification for either decision is the final product.
Outlines are Absolutely Not Necessary. At All.
You are probably a discovery writer, like Stephen King. And like me, and that is not a bad thing.
Check out this advice: Six Secrets of Writing a Novel without an Outline. Or this, from NY Book Editors: Planning to Outline Your Novel? Don't. Which contains many reasons why discovery writing is better. Or this one, Why Discovery Writing Is The Best Writing Method Ever, which contains even more reasons.
Which type of writer you are doesn't seem much of a choice; for me, if I outline a story I am pretty much done with it. I feel I have expended all my creativity on that, told the story, and I'm done with it. Writing out all the details then feels like a chore, the little creativity it affords is not enough to keep me interested in plodding through it.
With discovery writing, I do write a great deal I end up discarding or cutting, when I find the twists and ending I also have to do many more edits of the first draft. My foreshadowing has to be done after the fact, I read through and find places where it will seem natural (there are always a few). I adhere to the three act structure after the fact, meaning I identify where the key turning points are in the story and adjust the length of each act to fall where it should, it is what readers intuitively desire in a good story. You really have to embrace the mantra of "kill your darlings", cutting out scenes or characters you loved when you wrote them, or expositions you thought were going somewhere, that really just don't have a good place in the final plot and story.
This is NOT "efficient" in terms of knocking out a novel in three months; mine take seven or eight to finish. Forever abandon the thought you will not write a chapter that doesn't get published, or anything like that.
But for me it creates stories I love, and loved writing. Plodding through an outline will never do that for me. That is just a chore every day, while discovery writing is an adventure every day, exploring new territory and puzzling out what each of my characters -- and they feel like real people to me -- would actually do next to reach their goals. How they would react. How they would feel. That's enormous fun, and it is why I enjoy writing.
It seems that you don't want to use outlines because you end up changing them.
But the benefit of having an outline before writing your story is because if you want to change something, it's easier to delete and rewrite a few lines of an outline, than entire pages of your story.
Additionally, it's quicker to find plot holes or mistakes. And you could solve them before writing yourself into a corner.
It's never bad to go with the flow. In fact, there's a word for people who write like that. Pantsers. Those who write by flying by the seat of their pants. Writing an outline makes you more of a Plotter. Both have their plusses and minuses and as with most things writing (point of view etc.) it's a matter of personal preference and what works for one writer may not work for another.
I personally like to have an outline. If I get stuck in one section, I can look through my outline and, with an end goal discover what I need to do to finish that section. Or, if I get really stuck, I can jump to another but, the risk with this is that by the time I get to that section, I need to make changes to make it work with what happened before.
Most writing courses I've experienced follow the mantra of "when you fail to plan, you plan to fail" and while I don't always agree with this (who needs to plan two paragraphs for a 40 line poem?), I do see its benefit at certain times.