A mantra is a kind of blessing from the buddhas and bodhisattvas, and reciting it is a form of supplication. It’s comparable to the password you use to withdraw money from an ATM. You wouldn’t say that the password or the ATM has some kind of mystical power; it’s rather that these are the things you use to withdraw some money. The password is like a mantra, the money in your account is like your spiritual practice, the ATM is like the buddhas and bodhisattvas, and withdrawing the money is like receiving their blessings. Successfully withdrawing the money is the most important thing, not the password; so there’s no need to speculate about the password’s mystical powers.