Additional Contributors in Central is where you can enter and view performers and instrumentalists (Performing Contributors) and engineers and other Non-Performing Contributors.
It's best to think of these fields as representing what would normally go in liner notes on physical products; individual members of the band and their instruments can be outlined, as well as other studio contributors such as the mixing and mastering engineers and a variety of other studio personnel. Ex:
Contributors can be entered multiple times if they have contributed in more than one role. For example you may enter an artist like Bibio as the Main Artist and Stephen James Wilkinson as the Writer and Producer on the primary track metadata tab, and his other roles in Additional Contributors like Vocals, Guitar, Bass Guitar, Synthesizer, Sampler and Alto Saxophone.
As would be customary in physical liner notes, it's recommended to enter one individual per contributor field and use legal names (Saul Hudson rather than Slash, for example), but not required.
While these are optional fields, it is strongly recommended that you collect and submit this information on new products and use Change Requests to update older products. These fields provide even more info to downstream algorithms and voice search and are even displayed by some music services.
This is particularly important if you're submitting Jazz content and will eventually be required on all newly submitted Jazz.