Here are the reasons:

Your muscles "remember" everything they do, good or bad, right or wrong.

Excess tension in the muscles during practice is also "remembered". It prevents anyone who has it from controlling the muscles so that the correct notes are played in the correct way

All students have excess tension in the muscles when they practice

Any student who is making little or no progress is unaware of this excess tension, and is simply re-enforcing this tension every time they practice.

The only way to teach the muscles to play without excess tension is to learn what correct practice really is, and how it connects us to that tension we are creating but unaware of.

Once we establish the condition of "no excess tension in the muscles during movements" we must use CONTROLLED, CORRECT REPETITION to re-enforce the movements.

This repetition must be gradually done at faster and faster speeds WITHOUT INTRODUCING EXCESS TENSION in order to build our playing ability to where it needs to be to play well, and to sound professional.

All students, left to themselves, will practice too fast to actually achieve this. They have no idea of how slowly practice must be done in order to really bring results.

The metronome, when properly used, FORCES us to play slowly enough to achieve real, lasting results. It also allows us to gradually increase the speed of our repetitions by very smaller increments, much smaller increases than we could make left to ourselves. The muscles NEED these small increases in order to learn effectively.

. When we do enough CORRECT REPETITIONS of any passage we are trying to learn, we will MASTER IT!