A lot of the combine is simply testing athleticism. While a blazing fast 40 time does not have many direct applications to the game of football, it does act as a comparable measure of athletic ability. Think back to when you played high school or college ball and I'll bet that the kids who were the fastest at their position were generally also the most athletic in other regards such as jumping, acceleration, quickness, etc. The saying goes, "You can't teach speed" but really it extends to the idea that you can't teach a guy to be a good athlete, he just is or isn't.
Is athleticism the only thing that matters when in comes to NFL success? Absolutely not, but a lot of times it is the only thing the separates a Miles Austin from a Sam Hurd. Athleticism matters, that's why they do the testing. Having your head on straight also matters and that's why they do one on one interviews.
The combine is not pointless, the 40 yard dash is not pointless, the broad jump is not pointless, etc. The different elements of pre-draft evaluation have to be looked at holistically.