When building custom vehicles of any kind (including motorcycles, cars and even rc cars, sometimes you just want the "base" of the vehicle, i.e., the chassis, the suspension, the wheels and so on, and you may not in particular want the engine (since you want to install your own) or a body. So many companies will sell you a rolling chassis, consisting of the base mentioned above, allowing you to customise the remainder of the vehicle to suit your own whims; rather than having to deal with an engine or body you didn't really want to deal with.