teach correct movement

Just because you don't know how to coach a movement doesn't mean the person can't do it.

Nowadays we have an entire industry created by people who confuse their inability to coach a movement with the person having some mobility or injury issues which can only be fixed by elaborate "activation" exercises and the like, completed of course with lots of pseudoscientific jargon. 

Of course, "shove your knees out" is less exciting than a book full of mobility exercises.

A personal trainer's job is to teach correct movement. If you cannot get a healthy adult under 50 to an unloaded below parallel squat in ten minutes, and if after six months you have that person deadlifting 30kg, you're incompetent.