The conscious mind is projected into outside world through the senses. Meditation is essentially a process of turning the conscious mind inward into its sub conscious layers so as to flush out the negative forces accumulated in the sub conscious layers of the mind. The conscious mind which has projected itself outside has no way to turn inward into itself unless there is the process of centrifuging.
Continuous repetition of a benign and highly positive word centrifuges the conscious mind just as a whirl makes the water plunge into to the depths. However long one sits in meditation closing the eyes, the conscious mind does not find means or ways to turn inward., instead it just wanders continuously jumping from one thought to another. Off course there will be some relaxation because of closing of the eyes and cutting of the mind from the visual world.
Beyond that, the conscious mind cannot dive deep into its sub conscious layers.
It will be like swimming on the surface of the water in a lake without taking a dive to go deep into the depths of the lake.
If a benign word (mantra) is repeated constantly and comfortably, the conscious mind rotates around the word that is being repeated, and the centrifuging process soon takes place and it starts plunging into its own deeper layers. Thus meditation results in going deeper and deeper till the conscious mind reaches the bottom most layer which is in touch with ones own pure consciousness, just a sea diver reaching the bottom of the sea were the sea is totally silent, like the pure consciousness.
Therefore it is absolutely necessary to repeat a benign word (mantra) in a true and right meditation process. This is the science of RIGHT meditation process.