Didn't someone already post the incentive? If you're not a tuner I can understand how it may seem useless to you, but the fact that we had to jump through 26 different menus just to test out the tune we just made was kind of annoying. That it's now easy to access is practically godsend for that group.
To put it simply, there is no reason that test drive should be substituted in place of custom races, There's no point in that. The Point of it is to test out your vehicle, and now that it's there, it's much easier to do so without having to jump through so many hoops.