Why do some people feel the need to announce they are going to block you before they do it?

the same reason people announce they got blocked idk

I haven't done it, but one optimistic (but probably rare) reason might be to show respect to the other person.
If they were arguing with someone and posted a last reply right before blocking, the other person wouldn't see it or be able to respond (I think). So, it might look like that other person couldn't fathom a good response if they don't say that they were blocked.

Blocked for insubordination.👺

to feel superior

I'm going to block you.

Drama.

Yeah it does seem to be the norm to do that. I think it's like having the last word in an argument.

People love having the last word

quickly type back: Not if I block you first.