Volumetric comparisons end up grossly underrepresenting how much a billion really is. Human brain doesn’t really grasp the magnitude of a difference between a cubic centimeter and a cubic meter.
What we can immediately grasp is a difference between walking across a small parking lot and driving for a bloody hour.
That’s not how evolution works. Traits don’t evolve “to be useful”. Anyone who claims a goal to evolution has failed to grasp evolution.
Evolution converges on local maxima by selecting for traits that are good enough to continuously propagate through the filter of individuals death. For sexual reproduction, if a trait is not bad enough to continuously reduce carriers’ presence in a mating pool, it can and will remain.
It’s survival of not inadequate enough.