When you sing the music in your head while playing, your brain gives orders to your lips, fingers and the rest of your body to produce what is playing in your head. It will send signals to your muscles and after a few times playing like this, will find the most efficient way to do it. Of course you have to know your music. We very often say: "if you can sing it, you can play it" . It is true assuming that you have the strength to reach the notes of course.
I personally find after a while doing this that my lips start feeling very good and my tone improves a lot. I also have more endurance. I believe the reason for that is that the brain is acting to produce the desired notes and sounds rather than reacting to what it hears. The brains no longer needs to correct each note's tone and pitch after it hears them thus saving energy to play longer.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.