Funnily enough, aside from creating a surface for caramelization and adding flavor to the exterior of the meat, not much. Make sure that if you're marinating meat in a highly acidic marinade (e.g. one with lots of vinegar or citrus juice) not to exceed 24 hours of marinating time. The acidity helps to tenderize meat to a degree, but over time will completely unravel the proteins and lead to a mushy consistency. Ew!