why? because you don’t want to see certain things in your feed. people come hwre for various things. some people come here to mentally relax, so they don’t want to hear about negative topics like wars or people Fighting all the time. other people just are not interested in certain topics or posts but have them flood their feed.

example: i come here for relaxing mentally and to see posts about topics I’m interested in. so I don’t want to see negative stuff like causalities in wars, hearing people complain what bad things they experienced this day, what stuff has gone wrong in their life and other negative stuff. another example is lemmynsfw where I don’t want to see males showing their genitals to me since I’m interested in women, not guys. so i block communitys who are about such things.

short: because people don’t want ro see certain stuff. that’s why there is a block function. lemmy without a block function wouldn’t be useable since you get flooded with tons of posts you aren’t interested in by default without any blocks.