The Maketoys G2 Optimus Prime Trailer upgrade provides Optimus with an oil-tanker, just like the original G2 Laser Optimus Prime did.
And again just like the G2 Laser Optimus Prime, the Maketoy G2 Optimus Prime Trailer transforms into a battle-base and provides Optimus Prime with a.... Laser Sword.
But unlike the G2 Laser Optimus Prime Trailer, the Maketoy G2 Optimus Prime Trailer combines with Optimus Prime transforming him into a reinforced bad-ass version of himself. We should probably take the time to note here that the Maketoy G2 Optimus Prime Trailer also takes the time to replace G2 Optimus Prime's head.
But the real story on Maketoy's site is this. Do check it out!