FAQ:

"Can you not just block them from the chat log?"

Yes, however, this isn't helpful in a lot of situations. The issue with the chat log is that you still have to actively go out and find that player. If, for a plethora of reasons, you didn't block them on first interaction you cannot block them unless they whisper you again / you see them out in the world. So, say someone logs off because they're experiencing a panic attack, and don't block that person because of the interaction they had at that moment? You have to go out, find them, and then block them through the chat log (or targeting, I believe). Likewise, if you see a problematic person you don't want to associate with through other platforms or have been witness to a friend's mistreatment (so you want to block them)? Have to go find them! Want to block someone's alt after they mistreated you? Have to go find them!

Basically, any scenario -- and there are lots of them -- where you can't immediately block them through the chat box, right that second, means that you have to go out and locate them.