Once a player leaves, normal AI replaces it. Players usually put AI gold and exp low (players killing it get less gold and exp), while setting AI hp high (it has more health then obviously). However the worst thing is that replaced AI buys stupid items... Like Theugeris's hat... My most hated item as 90% of noobs and all AI buys it for unknown reason. I understand why AI buys it, but players... Well anyway. It sells good items for bad ones.
Enhanced AI obviously doesn't do that. Sometimes even buys better items than the player. It is not very great when it comes to some important decisions, but it clearly knows enough basics + buys upgrades, so team can very likely forget them and focus on themselves instead.
Without Enhanced AI, if the game is not decided yet and player leaves - then it's pretty much decided that team with AI loses.
With Enhanced AI - it is still unknown and the side with AI could win.