The benefit is overall harmonics, sharing of adsr feel (compression), corrective eq. In other words glueing together the track due to shared sonic characteristics.
Quick tip:
For each decide before what you want the song to sound like. More top end, more warmth etc. After choose each part and keep your goal in mind. Make sure you consistently A/B and make sure B is not louder. That way you hear the difference without it being louder. Don't bother trying to make them equal loudness because it is easy to fool yourself. Just make your adjustments a little less loud.