When writing essays for History class, 5 points are taken off for every contraction. I don't see why writing it fully out is better than using a contraction, since it's shorter and it doesn't affect your argument. Is this rule just because it's more informal, and should contractions be avoided in all formal writing for school, and in things like resumes?
This rule is because it is easier to impose simple rules than to inculcate good taste. In real life, try to develop good taste by reading excellent examples with attention. In class, do what you are told so you can get good marks, graduate, get a good job, and be able to afford to buy good books which you can read to improve your taste.
In the larger picture, be aware that many so-called writing rules have nothing to do with effective communication. They are shibboleths -- a form of virtue signalling used between members of a group to recognize others like them so they can exclude outsiders from preferment. If you want to enter a group that protects its membership with shibboleths, you will have to learn to imitate those shibboleths in your interactions with the group.
Different groups have different shibboleths. This may include different teachers, different schools, and different employers. There is no one universal key that unlocks all doors. You have to learn what each group's shibboleths are in order to gain entry to that group.
While I do see avoiding contractions completely as fairly arbitrary for expository writing in a History class, in a composition or creative writing class, there is a case to be made for seemingly arbitrary rules such as "no contractions".
When we learn the craft of writing, it is helpful to take us out of our natural conversational style. A conversational style can be appropriate for many kinds of writing, but developing the skills to write in other styles is at least helpful, and in some cases it's critical.
I can think of three types of writing off the top of my head where attention to seemingly minor details, like contractions, is very important: lyric writing (poetry and song), dramatic dialog, and political speeches. I'm sure there are others. Potentially all writing can be improved by attention to word choice, but details of word choice can be especially critical in these three categories.
In all three of the above cases, mostly because they are spoken aloud and heard, as opposed to read, tone and meter are highly important. By tone, I'm referring to the phonological content of each word. By meter, I mean the pattern of strong and weak syllables.
Contractions share a few tonal qualities that can be problematic. Contractions are formed by eliminating vowel sounds and compressing two or more consonant morae into one syllable. In some cases, two or more unvoiced consonants are put together (e.g., it's and let's). Consonant clusters can slow down singing and speaking and also create phonological noise and ambiguity. Consider both the auditory and lingual physical experiences of the following ways that say approximately the same thing:
- Do not
Don't is the hardest of the three to say (barring a speech impediment), because of the contraction, which places the voiced /n/ right against the unvoiced /t/. To listen to, it could be considered harsh and creates a stop in the sound because of the /t/. Do not changes the initial vowel sound to a longer one and adds the vowel sound between the /n/ and /t/, which helps it flow a bit better. Never is included as a contrast because it is completely voiced and includes no stops.
Next, consider the meter of the following four phrases that again, all have similar meanings (meter for each phrase is indicated in parenthesis):
- Don't think (strong strong)
- Do not think (weak strong strong)
- Do not ponder (weak strong strong weak)
- Never ponder (strong weak strong weak)
From a metrical standpoint, don't think is bordering on a disaster. A third strong syllable would make the phrase sound like a fall down some stairs, and don't think alone is a bit of stumble. Again, the unvoiced consonants of /t/ and /k/ slow things down, and the cluster of /n/ /t/ /th/ is bound to displease speakers and listeners alike. (On the other hand, you might be able to get a fun little EDM beatboxing started with just /nt'th/ /nt'th/ /nt'th/ /nt'th/.)
Never ponder has a much more musical trochaic flow. Tack on another two to seven metrically related syllables and repeat at least part of the line, and you've got the beginning of a song or persuasive speech. There is only one unvoiced consonant, /p/, and the stop there can be effective in strongly establishing the pattern of two trochees.
Finally, even though we got here by discussing sound, note the stronger and more nuanced meaning of never ponder versus don't think. Not only are contractions fairly "weak" words, their expanded forms are rarely much better. By being forced to completely eschew contractions, you are required to more carefully consider your word choices. Teachers of writing may often impose such arbitrary rules, not to annoy students, but to force them out of their stylistic comfort zones and encourage them to more carefully consider each word. I once took a class where we were asked to never use any form of the verb "to be". I learned a lot about writing style in a very short time.