Money ! (obviously) How much they charge for a lesson will depend on the going rate in their area, and the length of the lessons that they give. My students charge half my own rate, rising a bit when they have passed Grade 8.

Experience and confidence - development of communication skills.

Scale-fingering becomes much more secure when they are having to teach it.

Sight-reading becomes more assured when they have to play aural tests and demonstrate pieces.

Singing becomes more confident when they have to coax a reluctant singer to do so in aural tests.