Why are some songs featuring other singers, but the group it is actually by didnt sing at all? (Read description)

Ok so this might be hard to explain, but I can't find it anywhere on Google. But some songs, are by a group, and feature another singer, but the person it is featuring sings everything. For example This is what you came for. It is by Chainsmokers, and is featuring Rihanna, but obviously Rihanna sings everything. Another is Dont let me down. It is also by chai smoker, featuring Daya, but Daya does all the singing. Whyyy!?!?