Yes. It allows you to impersonate someone, cause issues for other players under their name, sell them on to others to want to do bad things like that etc.

In Roblox's case there are items that sell for thousands of dollars. In Minecraft's case there's a very large marketplace for the accounts themselves - the game has a lot of cheaters that get banned from servers regularly and cycle through accounts to keep playing.

You can then steal their account balance and any items you have.