This is, no doubt, one of the most important sutras, because here, Patanjali does nothing less that giving a definition an explanation of the benefits of yoga.
Usually the definition given for yoga is « union », that’s how it is define in the Bhagavad Gitã and in some of the Upanishads, but he never talks about that in the sutras. Instead he defines yoga as the fact to quiet the mind in a way.
Because of how we think, and how we perceive the stimuli from the outside world we are shut down from awarness, by practicing yoga we can still the and the body and therefore we might have access to pure awarness. Yoga was used traditionnaly to prepare the body for meditation, and whant is meditation ? It’s a practice where your body and mind are steady, it.s a place where you concentrate to find awarness.
Patanjali thinks that human beings can’t really make a difference between cousciousness and Pure awareness , so that’s why he said that we have to allow Counsciousness to settle from the whirling thoughts, sensations and emotions to be able to reflect pure Awarness. Yoga is a journey within ourselves, that will help settle down the agitated patterning of consciousness.
And it’s only when it happens that we can reach pure awareness.