Game mechanic purupse of xd6 roles

[sekalo]

At first I was confused when to use modifiers or the xd6 rolls; however, I have been pushing forward working with my group and TFT powered game.

However, I started to wonder what purpose in game mechanics does xd6 provide over the -x bonus/penalty modifiers on rolls? It seems like skills/talents use the xd6 dice rolls while combat uses the -x modifiers. Why not keep it uniformed and just use one or the other.