The Autobots gave the weapon to the humans, so they could protect themselves better against Decepticons. The Axion weapon itself is a big gun, able to create a shield around it so it can't be damaged.
Note:Though the Axion weapon resembles much of the looks of the Rail gun, it is unknown if it is the same weapon.
In the Autobot campaign it was the mission of the Autobots to protect this weapon while it was transported by a truck. In the Decepticon campaign it was te mission of the Decepticons to destroy this weapon, now fully active.