Yeah, you bring up a good point about the way most people talk about freewriting. For whatever reason, we get introduced to it as if it belongs in the "prewriting phase" of writing.
The reality of writing for me is that you can do whatever you want behind the scenes to make things happen. Writing is taught as if there are necessary stages, but I don't think we should stick freewriting into that phase with the assumption that it belongs there.
It's a technique, so it can be used at any time.
I like to freewrite each morning. I find that it helps me clear my mind and to come up with some goals for the day. It can be surprisingly beneficial for just bringing about a general feeling of lightness for the rest of the day. How exactly that happens from just freewriting a couple of pages each morning must be difficult to describe. I am not sure if I really understand how it works. It's subtle, I guess. I'll say it that way.
I also freewrite to generate new material. I start wherever I happen to be and then I keep going until it feels finished or I feel like moving on to something else.
I keep a file of jots and notes that I look through to see if any of them grab my interest. If so, then I will freewrite to expand them.
Freewriting is also great for gaining some additional understanding about something. A goal I have or a writing project I want to figure out. One way I do this is to hold a topic or a question in the back of my mind. I'll write and basically talk to myself. It's way more productive than thinking about it. Maybe the material can be used in the actual document, or maybe it isn't usable except as part of the process.
Tons more I could say on this. It's a remarkably versatile way to write and think. Peter Elbow is a great resource for further study of the philosophy that underlies the technique.
I also teach courses in freewriting. I would love to see more people adopting freewriting outside the classroom. I have found it transformational, especially when doing writing marathons.