The essential nature of that which is seen is exclusively for the sake of the seer.

Bryant Commentary:

The seen, that is, the knowable—buddhi (and ultimately prakṛti herself)—exists only for the sake of puruṣa, who is the seer, reiterates Vyāsa, and is thus dependent on another, not on itself. As II.18 informed us, the purpose or function of the seen, prakṛti, is to provide either experience or liberation to puruṣa, and this purpose is fulfilled when experience or liberation has been attained. The nature of experience consists of pleasure and pain, and pleasure and pain are not conscious of themselves; they are experienced by an other. This other is puruṣa. Therefore, the purpose of the seen is not for itself but for the seer, just as the purpose of a bed, continues Vijñānabhikṣu, is for the sleeper, not for itself. Or, as Vācaspati Miśra puts it, the relationship of puruṣa and prakṛti is like that of the king and his possessions.

Since prakṛti has nothing more to do once its purposes are fulfilled, asks Rāmānanda Sarasvatī rhetorically with an eye to the next sūtra, does this mean that, deprived of its function, prakṛti would no longer be perceived? Might it even cease to exist?

To Serve As Objective Field To Purusa Is The Essence Of Nature Of The Knowable.

In essence, the phenomenal world exists to reveal this truth.

For the sake of that Self alone does the seen world exist.

The very being of the Seen is for his sake (i.e. Prakrti exists only for his sake).

Nature and intelligence exist solely to serve the seer’s true purpose, emancipation.

Iyengar Commentary:

Intelligence exists to serve as the seer’s agent, to free the consciousness from avidya. The natural tendency of all the soul’s agents – mind, senses of perception and organs of action – to be drawn to and identify with the sensory and phenomenal world is to be avoided by discrimination, a faculty of intelligence. Uninterrupted yogic sadhana will help us overcome these obstacles and allow the soul to reveal itself.

If the sadhaka slackens in his sadhana and becomes inattentive, the senses disturb the seer and he is caught again in the pleasures of the senses. This study of mind and investigation through intelligence is the innermost quest: antaratma sadhana.

This sutra conveys that consciousness, the essence of nature, which is cognizable, exists for the sake of the seer who sees to see.

The seen exists only for the sake of the Seer.

Satchidananda Commentary:

As we saw in the previous sūtras, nature is here to give experience to the Puruṣa and so we think the Puruṣa is doing the experiencing. In reality, the Puruṣa isn’t experiencing anything. It is just a witness. But since it appears to be experiencing, we must try to understand it from that level. Once we go further, we will realize that the Puruṣa is neither the doer nor the enjoyer and we will change our vision and attitude; but, for now, we start from where we are.

The very word “understanding” is a combination of two words: “under” and “stand.” To understand, we should stand under. But stand under what? Under where we now stand. We should know where we stand first and then try to “under” stand, to go a little deeper. When we try to understand, we will find we are not all on one “stand” but at different levels, with different capacities, tastes and temperaments.

Each individual has his or her own stand. My understanding is completely different from yours. One and the same scripture appears differently to different people as each one tries to interpret it from where he or she stands. So, here Patañjali is saying we are now under the impression that the true Self is experiencing something, but one day we will know that the Self never does anything nor will it ever enjoy anything.

The nature of the experience is for him.

SV Commentary:

Nature has no light of its own. As long as the Purusa is present in it, it appears light, but the light is borrowed; just as the moon’s light is reflected. All the manifestations of nature are caused by this nature itself, according to the Yogis; but nature has no purpose in view, except to free the Purusa.

The knowable comes out as the object of the Purusa appearing as an act of consciousness. For his purpose only, therefore, is the being of the knowable. The meaning is that it possesses the nature it does, for that purpose. Its nature, however, depending as it does upon another for its existence as such, is not cognized by the Purusa when the objects of achieving the puposes of enjoyment and emancipation are over. This would seem to imply that it is destroyed, because what constitutes its very being is thus done away with. But it is not destroyed.