The Rumble Bike will be used when you find Manny in the end of the game. Because you will need to get back home as fast as you can because Manny and Greg’s monther will return.
How to Get the Rumble Bike
Go out through the window, across the roof to the left and run left up the hill to Rowley. Talk to Rowley and then Manny appears on a rumble bike and runs past you. Run back down to the right after Manny and go to Main Street. Then go past the Fast Mart and you’ll find the rumble bike stuck in the snow. Pick it up.