Sometimes singing teachers say things like: use your ears and listen when you sing.

To me this sounds a bit crazy. I would say that what most people do is focusing on feeling the notes rather than on hearing them. I am not the expert so I might be wrong. I myself try to feel the notes rather than hearing them. This is why I don't understand why focusing on hearing is the best thing to do. I am not sure that singers even hear what they sing. Pianists might hear but not singers.
What does a singing teacher refer to when he/she says that a singer should use the ears and listen?