Because the muscle it treats is the one that breaks teeth. Patients arrive complaining of morning jaw ache, temple headaches and a jaw that is tired by lunchtime, and are found to have flat wear facets, cracked cusps and a square jawline from muscle hypertrophy. That is one problem, not three.
Relaxing the masseter reduces the force available, which relieves the ache within two weeks and protects the teeth and any dentistry on them. There is a cosmetic consequence too — the jawline narrows over two or three treatments — which some people want and others do not, so we discuss it before rather than after.
It is not a replacement for a night guard, and we would almost always do both: the injections reduce the force, the guard takes what is left. Together they are the difference between replacing crowns every six years and not.