If someone follows you into every single topic to shit on you no matter what you're actually posting, you can ignore them but everyone else is still going to see them attacking you, and they can essentially accuse you of all manner of things that you now can't defend yourself against because you can't see them. You won't even be able to mark them for harassment (which would be perfectly valid) because you don't even know it's happening.
The idea of block is that you essentially remove yourself from them even knowing you exist, with the assumption that they'll eventually get bored of posting about you and move on with their lives.
In practice, it doesn't really work because most of the really shitty trolls or obsessed harassers have multiple accounts anyway, so they'll just log into a different account to see what you're up to. Or will just make shitty little snide comments about you every now and then in entirely unrelated topics just because they know you're still out there even if they can't see you.
Actually no it isn't. Block is supposed to stop the person that blocks from seeing people's posts they don't want to see. It is NEVER supposed to stop somebody from seeing other's posts. ---