I do the same. It simply means theres less accidental leakage / self-doxing that could be pieced together if you (or llm) read every comment on the account.

Suggestion: Pick a long term account, dump the comments, and see what an llm could figure out about the target

I do it sometimes just to restrict my own pride in the account. I get a buzz from upvotes and that upsets me on a deeper level.

Same, but also for the opposite reason: a new account gives me a chance to do better. If I post lame comments, I accept the lameness of the posts attached to a particular user name and the hesitation I feel to post more lame comments decreases. With a fresh identity, I am more likely to avoid lame posting sort of like how you avoid going out in the mud in brand new sneakers. A sort of repentance; being born again in the digital realm.

You can build quite an extensive profile of someone given enough post history. More post history means more details. Especially nowadays with LLMs it's trivial. This can lead to all sorts of issues. One is people I know in real life being able to identify me. Another is that through various means my account may be linked to my personal identity (e.g. through matching usernames or emails across platforms) and oppressive regimes (now or in the feature) may use my post history to take action against me.