Classical Yoga serves the three “spiritual” dimensions of embodiment—adhibhautika (five elements within), adhidaivika (inner celestial forces), and adhyatmika (Self).
Japa nourishes and purifies the subtle body just as food and medicine nourish the gross body. Nama japa includes chanting OM, and requires no prerequisite.
Dhyana pacifies the causes of suffering, purifies chitta, & normalizes a disorganized mind. Sages used meditation & dhyana to attain jnana (spiritual knowledge).
Japa is a form of dhyana that everyone can do to nourish the subtle body. Manas is the “tip of the iceberg of chitta.” Dreamless sleep restrains manas (mind) but not the chitta vrttis.