Crates are random power-ups found throughout the game, they give random bonuses, and sometimes none at all. SOME of the crate drops are
Nothing (Soldier that retrieves crate always recieves +3 exp)
A free soldier
+5 Exp to all soldiers
+5 Rifle to all soldiers
+100 Armor to vehicles
You cannot get anything "bad" from a crate, but if germans pick it up, they will get more troops automatically.