I can think of a couple of uses. Well, basically just one use. You could hide posts that cause distress for whatever reason. For example, I hate snakes and if someone posted a neat picture of a snake I'd probably hide it, just so I don't have to keep seeing the same post and jump scaring myself with a picture of a snake while scrolling. Another example might be a thread in which people are arguing and you don't want to get dragged into the argument. I actually literally did this earlier today with a thread where people were yet again arguing about the upcoming US election and whether voting Biden is a reasonable choice. I've read this argument so many times and it's so tempting to jump in and be an asshole to someone who is wrong and I just don't want to do that today, so better to hide the post so I don't keep seeing it while scrolling.

So yeah, you can hide posts you don't want to keep running across while scrolling for whatever reason. Seems like a pretty useful feature to me, I'm glad we have it now.