Todd Howard said that he thought of VATS during development. But in FO3 Agility is used only to determine the number of APa to use in VATS. So...what was it used for before?
In FO3? Probably just determining move speed and relevant stats/skills. Before that? Derived stats like AP and AC. Bethesda may have had other ideas for using AP in FO3 before Todd Howard's half assed VATS came into his head.