Because it is the universal primary sacred sound of the universe, the one that makes us all one.
Pronounce it with intention, visualize it with intensity and focus on the point in your body or your mind that needs healing, the results can be awe-inspiring
Heat and movement brought sound into this universe and there came life at the beginning
That is why it is sacred and it is pronounced by all the yogis of this Earth. It unites, it heals, it spreads positivity all around a yoga class, and all around this Earth.
Transformation will come as healing if you practice OM regurlarly, even if you listen to it.
The sound of OM has a direct effect on the waters of your body which are greatly influenced by the words that you say and the sound that you make.
Cf: Emoto Masaru “Le pouvoir Guérisseur de l’eau ».