Does marinating chicken make a difference?

Yes. It can prevent pancreatic cancer if the chicken is to be cooked on BBQ or very high temperatures. Extremely high temps produce carcinogens known as HCA and PAH. Marination prevents those carcinogens from forming.

If its not cooked on BBQ, then I don t think it makes much difference.

Not as much as a lot of people seem to think because the marinade doesn't penetrate into chicken meat very well. Boneless chicken breasts that are pounded to a uniform thickness accept marinade much better because the pounding breaks down the meat fibers and allows the marinade to penetrate. It also cooks much faster so don't over cook it or it will dry out (common problem for chicken breasts).

Depends. You have to read the label.  Some chicken says something like "Contains up to 7% solution to enhance flavor yada yada yada".  That chicken has already been marinated with no one knows what, and you would have to dry out the bird in order to get it to accept any more brine. If the statement is not there. you can somewhat tenderize and certainly flavor the bird by marinating. That practice is used by processors to sell salt water at a couple of dollars a pound.

If you want a different flavor rather than just 'plain' chicken.

a difference with what?

You taste the seasoning more in the meat