Crates serve different purposes in Modern Combat games.
Modern Combat: Sandstorm[]
The Weapons Crate is a box that is used to obtain weapons in Modern Combat: Sandstorm.
Modern Combat 3: Fallen Nation[]
The ammo crate in gives the player ten mags of ammunition for all types of weapons currently equipped.
The grenade crate in Modern Combat 3: Fallen Nation gives the player two fragmentation grenades. It can be found during many of the missions.
The missile crate in gives the player [how much?] missiles.
Modern Combat 4: Zero Hour[]
In some missions, crates containing guided missiles can be found for use with the CTK-88 Crumplor. So far it's only found in 01: Red Summit.
Modern Combat 5: Blackout[]
Crates can be found and used to refill your ammunititon. This works for any weapon.