If someone is harassing you on Twitter, muting them doesn't stop them from interacting with your tweets. You don't see it anymore, but people looking at your tweets still do, so now people see things attached to your tweets you have no idea about.

Some also argue that its better someone is explicitly told "you can't interact with this person" vs them trying and it getting hidden silently and them having no idea they can't reach you that way, and thus suggest blocking over mute also for people who didn't attack you. (A "don't show this person unless they directly talk to me" would be nice to have instead)