Twitter is writing practice. Can you condense an idea, and express it in a pithy, vivid, engaging way? Can you slip in a bit of alliteration, rhythm, elegance, poetry? Can you use your vocabulary for precision and concision? Can you persuade? Can you entertain?

The character count adds a restriction that enforces discipline on the writer. The lack of an edit button makes your misstatements (or outright wrongness) more consequential.

The hostile mobs teach the writer diplomacy and delicateness. They also make the writer choose who to ignore and who to engage, thus teaching discernment. They force one to know (or curate) one's audience.

The quick feedback and the dopamine dose from the likes and the responses make it fun.

(Though most of the fun, I find, is private and unnoticed. I can't tell you how much phrases like "outright wrongness" and "King Canute knew" tickle me, or the fun I have with commas.)