Why do I start with a plain bike?
Just like anyone coming into MotoGP as a rider you have to work you way up through the ranks. So we start you off with a plain bike and then introduce you to sponsors who are interested in having you race a bike with their logo on it. Sponsorship is the commercial activity which enables motorsports (and any sport for that matter with the possible exception of Curling) to exist.
So you will see that you quickly move from your plain bike to a sponsored bike where that sponsor will pay you to race each week in valuable in game coins which you can spend on upgrades and other stuff. Of course if you want to bring the level of realism up a notch you can acquire a MotoGP Team bike or better still get a Pro Rider which allows you to support and race as your favorite rider. When you race as your favorite rider you are directly and personally contributing to their success in the Fan World Championship.