From The TimeSplitters Wiki

Crates are large boxes that can be added as decorations or Story mode objectives in Mapmaker maps in both TimeSplitters 2 and TimeSplitters: Future Perfect (although the story mode feature is only in TSFP). They are indestructable (TS2) and destructable (TSFP) depending on the game. In TS2, they are indestructable, although they can create a minor explosion once if they are damaged enough. In TS3 they can be destroyed and also be used in custom story mode missions.
Crates are rather short to be used as cover, unless you are playing short characters such as Monkey. They are also to small too completely cover up a doorway.