Add moisture to the meat? Preserve meat? On what planet?
Joy is right that some have them and some not but, no, not helpful as to why either. (I daresay many cooks don't think about the 'why' here.)
Oil is used (or should be, imo) for three reasons: to lubricate the surface of the item being marinated, to add or carry flavor (it adds its own, if not a neutral oil; it carries fat soluble flavors that are in the marinade), and to soften acidity. All of these things can be done in other ways, sans oil, and none are necessarily required.
Oil usually does not impede salt penetration but can impede flavor penetration depending on the quantity used and whether the marinade is emulsified. For non-emulsified marinaded with significant oil, the item being marinated can end up sitting in a lot of oil while the rest of the ingredients float elsewhere--not a good thing. Because oil in quantity will soften acidity it can seem to knock the top off bright, acidic flavors. This can be fine if the marinade is quite acidic and bright, not necessarily so fine if it isn't, so consider this when deveoping (or tweaking) marinades.
Many marinades I make do not need to include oil but I usually stick a little in--or another fat--so as to carry fat soluble flavors better. (This isn't always necessary--it depends on how much fat the item being marinated has on or near its surface.)
The key ingredient to flavor penetration is salt--and some time. Penetration of water soluble ingredients is fastest, as one would suspect, and is made more efficient (and faster still) by the use of adequate salt. CI does 'fast' marinades by adding soy sauce as a main ingredient because of its salt content. This isn't really my thing, unless I am doing an Asian-style marinade, because I am not one that often mixes food metaphors, i.e., I won't use soy sauce in an otherwise Mexican-style marinade, say, or one with balsamic and garden herbs, and so forth. But it isn't necessary, just the salt is. When using chimichurri as a marinade, e.g., I will often skip most of the oil (all but maybe a teaspoon) and replace it instead with water. Then I add a decent amount of salt, purée, and marinate. I rub the marinade off for cooking, oil the meat a little for lubrication, and cook. The chimi that is served alongside is made in the standard way, with the oil. In a way, this is similar to Bry's approach with RSC. The bright flavors are transferred into the chicken during marinating. The oiled baste is softened, but because it is used as a baste doesn't negate the brighter notes that have moved into the chicken, provided the chicken was marinated for sufficient time in the first place. ('Sufficient time' is usually less in oil-free or low-oil marinades.)
Oil, especially in emulsified marinades, does seem to slow overall penetration. Use this to your advantage if needing to marinate over some time.
A good replacement for oil when you wish something in there for the fat soluble flavors is dairy fat, especially yogurt or buttermilk. Either can be used with--or take the place of (importantly)--acids being used to 'tenderize'.
