I've been playing Dota 2 for a while now. I'm a level 6, so I'm still a noob, but I do have an idea of what's going on.
Anyway, I've been watching some Purge Plays videos. In these, I see him denying his creeps even if nobody is nearby that would last hit those creeps. For example, he is farming in an empty lane, nobody is nearby, nothing is showing on the mini-map, and he still denies a creep.
Why would he do this? Is it just an instinct or is there a good reason for doing this?