In Verse 28, Patanjali lays emphasis on the repetition and meditation on this Pranava mantra i.e. Aum. 

Why should there be repetition? We have not forgotten that theory of Samskaras, that the sum-total of impressions lives in the mind. Impressions live in the mind, the sum-total of impressions, and they become more and more latent, but remain there, and as soon as they get the right stimulus they come out. Molecular vibration will never cease. When this universe is destroyed all the massive vibrations disappear, the sun, moon, stars, and earth, will melt down, but the vibrations must remain in the atoms. Each atom will perform the same function as the big worlds do. So the vibrations of this Chitta will subside, but will go on like molecular vibrations, and when they get the impulse will come out again. We can now understand what is meant by repetition. 

It is the greatest stimulus that can be given to the spiritual Samskaras. “One moment of company with the Holy makes a ship to cross this ocean of life.” Such is the power of association. So this repetition of Om, and thinking of its meaning, is keeping good company in your own mind. Study, and then meditate and meditate, when you have studied. The light will come to you, the Self will become manifest.

But one must think of this Om, and of its meaning too. Avoid evil company, because the scars of old wounds are in you, and this evil company is just the heat that is necessary ot call them out. In the same way we are told that good company will call out the good impressions that are in us, but which have become latent. There is nothing holier in this world than to keep good company, because the good impressions will have this same tendency to come to the surface.