Why striking sports that have no grappling at all have weight divisions?

In boxing,mauy thia,kickboxing,teakwondo, karate, that does not allow throws or sweeps how does wight affect the fight if grappling,throwing or sweeping is not aloud. What if someone is skinny and tall with a long reach and someone is short and muscular or short and fat how does that affect a striking match doesn't the tall skinny person have advantage due to reach?