Cheap leaf springs?

Okay, I took the advice to avoid cheap leaf springs and I finally saved up and bought a set of 340 XHD springs with +1" lift from ESPO!
They asked if my eye bolt was 1/2" or 5/8". Said that some people drill them out for the bigger bolts. I cant help my curiosity, why do people move up in bolt size? Strength? Old bolt holes wallowed out?
Thanks!