What is the purpose of marinating chicken?

Marinating chicken serves dual purposes: 1) Adds additional moisture compensation so to avoid or reduce risks of drying out the chicken, primarily on a grill. 2) Be the chicken done on a grill or baked/broiled in an oven, marinating also enhances the flavor of the chicken--generally, the longer chicken marinates, the better.

Any method you cook chicken, it's recommended you make enough marinade and RESERVE SOME for basting PRIOR to marinating chicken. 

allows for the flavor to come out

A marinade is a liquid solution in which you soak foods, particularly meats, before cooking. ... An acidic marinade reduces the formation of these chemicals. Meat such as chicken breasts and pork loin are more likely to dry out on the grill and will benefit from a marinade to retain moisture.

too infuse the flavor in the chicken.

Marinating meat allows that seasonings to penetrate the meat and flavor the entire piece.

To make it eatable.  

To marinate chicken.

if you wanted to make something with chicken then you marinating it

To add flavor to the meat, in some cases the marinade ingredients makes / keeps the meat tender.

To make it taste better and to have favor