Crates serve different purposes in Modern Combat games.
Modern Combat: Sandstorm Edit
The Weapons Crate is a box that is used to obtain weapons in Modern Combat: Sandstorm.
Modern Combat 3: Fallen Nation Edit
The ammo crate in gives the player ten mags of ammunition for all types of weapons currently equipped.
The missile crate in gives the player [how much?] missiles.
Modern Combat 4: Zero Hour Edit
Modern Combat 5: Blackout Edit
Crates can be found and used to refill your ammunititon. This works for any weapon.