Grafting vinifera grapes on a resistant Rootstock is critical for long term protection against phylloxera in most regions of the world, including the eastern USA, if we are talking on a vineyard scale. If we are talking on a backyard scale, phylloxera susceptible varieties will sometimes do fine for many years, especially in soil that has never been previously planted to grapes. It’s a roll of the dice however. Some other things Rootstocks can provide in grape is adaptation to different soil types, calcareous soils vs. acidic, nematode resistance in some rootstocks, drought tolerance, some degree of vigour control, etc.
Rootstocks will do nothing to control fungal pathogens or pierces disease however, as those are not soil borne problems.