So people can pretend the "abusive" user (e.g. ones posting "harmful emojis") doesn't exist.
It's a mindset, I think. See, for instance, this woman complaining about News Genius[1], discussed here [2]. At one point, she's upset that a blocked Twitter user can see her public tweets.
People should be able to moderate and exclude people. It's just funny how powerful they think that capability should be.