Can you\why don't some restuarants marinade without chicken skin?
There's an overlap, and a difference, between a marinade and a glaze. If a mixture contains a fair amount of sugar, then it's probably intended more as a glaze, and the cooking method will usually be grilling, or roasting, (rather than pan frying). There's no reason not to glaze poultry flesh directly (teriyaki, for example) but the bigger the piece, the longer the  exposure to fierce heat will have to be to cook it to the middle. It's not easy to grill a whole skinless chicken breast without drying the outside before the middle is cooked. The combination of the fat from the skin and a glaze will protect and baste the flesh. The surface will be crisp, juicy, and caramelized, rather than parched and leathery.