i think the fun thing about writing outside your English classes is that you really *do* realize there's merit in breaking the conventional 'rules'. punctuation and the way you separate your paragraphs is also a form of storytelling and prose.
a paragraph being a single line gives it more weight, and, in the case of monologue and internal musings, it lays out the character's thought process better. they jump from one thing to the next.
abusing and technically misusing emdashes and hyphens makes things disjointed and can show stumbling, quick-paced thoughts. if there's a lot, it shows that they're not really completing their thoughts, jumping around, disoriented and/or panicked.
periods are also known as full-stops, so stopping a sentence abruptly with one can likewise convey stilted ideas, thoughts, dialogue, whatever.
an 'overuse' of italics shows a character stresses specific points and words a lot, which, depending on the character, reveals a lot about them and their personality.
at the end of the day, writing is also art and you're free to format however you want
