The 40 is often considered one of the better measures of general athletic ability. It's going to tell you less for certain positions, but when someone has this kind of number at that kind of size, they are making it clear they are supremely athletic.
Geno Atkins ran a very fast 40 at 293 lbs. If he were testing today, with all of the high tech training these guys do it would have been even better, I'm sure, but the point was that he had uncommon athleticism for a guy of his size at that time. It was one of the few markers for the untapped potential that eventually flourished here.