@escamoteur @mekkaokereke @markallerton @ks yes, but you have to see a harassing message to block it, which can be really traumatic.
If there are 100 people on an instance, and each of them has to block an harasser manually, that's 100 incidents of harm.
If the first harassment incident results in a site-wide block, then 99 of those incidents are prevented. It's still terrible for the first person, but the overall harm is less.