How you place your hands, bend your wrists, stretch your fingers and generally hold your instrument can greatly impact how you play it. Depending on the instrument, a simple bend of a wrist can lead you to consistently play out of tune, or make it hard for you to hit a specific note.

Even just touching strings in different ways can produce wildly different results; think playing a harmonic note on a bass guitar, which is how you get such a low-sounding instrument to play a high-pitched note. How you blow air through your horn can greatly change the timbre and mood of your music.

There are a lot more examples, and they are all difficult to correct if you aren't able to see what the musican is doing.

It's similarly essential for teaching voice.

The exception here is at the MasterClass level. Fuck the world for autocorrecting that to camelcase.

Physical matters of technique. Depending on the instrument or lack thereof: grip, placement of fingers, posture, breathing, lip embouchure, and more. Heck, you could even have it put together wrong or be holding it upside down. A remote teacher cannot easily convey this things in a timely manner without seeing the student.