The short answer is that marinades work to tenderize and add flavor to meat.
Meat is made up of strands of protein. And the cooking process works to break-down these protein strands – making the meat tender. But by using marinades you are getting a jump on the tenderizing process… while you impart flavor.
The enzymes and acids in marinades go to work breaking down the protein strands before the cooking process ever beings. And as the meat soaks in the marinade, it will begin drawing in flavors and seasonings from the solution.
Smaller cuts are ideal for marinating because the flavors are completely absorbed throughout. But larger, thicker cuts don’t benefit as much from a marinade. Your marinade will only effect the first few inches of the meat… and for larger cuts, this can make your exterior mushy while the interior remains dry and flavorless.