It's all well and good to hoard loot for yourself, but sometimes you want to share. Besides, who knows? Maybe your friend has a piece of gear that'll complete your set of armor and in so doing, complete your very soul.
In Torchlight II, it's very easy to trade loot with other people in your multiplayer game. It's all very casual and handshake-based, which feels in line with the game's generally relaxed attitude towards multiplayer. (This is, of course, one of the big differences between Runic's game and the auction house-based Diablo III.)