In the short, he's the rocket that Mater rides on to get to the Moon. He first appears, where he notices Mater towing a car and says "Well, that guy's got a tow." That made Mater decide to be an astronaut
Then, Mater gets on Roger, then Roger introduces himself to Mater. Then they launched into space, and Roger was having fun with his job. Then when they arrived on the Moon, Mater got offboard. Roger wishes him good luck.
"LET'S BURN THIS CANDLE!"