The practice of Mantra recitation is only one application of a trait of the human brain and how it functions. Namely, that reciting something aloud speeds up the process of internalization. Have you ever had a music instructor that made you sing your part instead of playing it? Same concept.

If you associate certain ideas with each other over and over again by force, your brain will start to perceive the association as correct.

"Mantra" has sort of become a linguistic shortcut for referring to this effect and its application.