Just, plain, fun.
The 2 player mode offers-up a new set of challenges, since some of the tools and abilities are split between the two characters. Lara's partner Totec has a shield and spear to start, while Lara has her trusty grappling line. Lara can get a boost by standing on Totec's shield or jumping across his spears while he throws them into a distant wall. Lara in turn has to pull Totec up to higher ground with her grapple and can even bail him out when he falls into pits.
There are various optional challenges to be completed in each level, as well as weapons, relics, costumes and upgrades to be both found and earned. Combined with the solid multiplayer, this game offers plenty of replay potential on top of being just plain old-fashioned fun.