Chests can be used to store various objects and collectible items. All chests on a lot share the same inventory, presumably to make object retrieval easier.
In Jungle Adventure, Selvadorada's tombs contain treasure chests that Sims can loot. Their contents are reset after every expedition, so there is always something new to discover. These chests can also be bought in build mode, but they do not contain treasure when they are bought this way.