You can go on line and read 1000 different opinions about marinating meat. Most read like this: "One of the simplest ways to flavor meat is to marinate it. That is, treat it like a sponge. All you have to do is pour sauce on the food, let it soak for a while and then cook it."
Personally, based upon over 30 years of experience in the kitchen, as well as in the BBQ pits, I don't believe that is true. In fact, even after hours of soaking, most marinades don't penetrate meats like beef, pork or lamb much more than 1/8". A side effect of marinade on meat is a wet surface, which impedes the ability to crisp or obtain a nice char. In short, 'soaking' marinades adversely affect the taste and texture of many meats.
On the other hand, meats such as chicken, turkey, and some pork cuts usually absorb a little more marinade (though not as much as we would like) and fish will literally absorb it like a sponge.
If the marinade contains a lot of salt or acid, they will affect the meat in a different way. Salt is important, as a flavor enhancer, and has good penetrating properties, as well as a conduit for pulling in other flavor components (like brine).
Acids can serve as a kind of marinade; fruit juices (i.e. lemon, apple, pineapple, orange and white grape juice), vinegars, and even sugar-free soft drinks can break down protein, which is a process known as 'denaturing'. However, too much acid, or even a small amount of acid over too long a time can make the surface of the meat mushy, which inhibits crispness or charring.