A basic marinade is acid, oil, herbs/spices. I understand the herbs and spices are for taste, the acid helps meat absorb by breaking down the tissue and taste. What is the oil for?
The oil is for carrying the flavors of the herbs and spices throughout the marinade (and ultimately, into the meat or whatever you're marinating). Some of the chemical compounds in those herbs and spices are more soluble in fat than they are in water, the oil absorbs those flavors and distributes them more evenly throughout the finished dish.