You need Light

When encountering the difference between Chakirah (Jewish philosophy) and Kabbala, one can ask the question: what’s the purpose of Light? After studying Chassidus, one can ask a more eidel question: if everything is about the Essence of G-d, and, indeed, only about this world, again, what’s the need for Light, intellectual understanding of the deeper aspects of a mitzva and emotions while performing the mitzva? If we reach G-d’s Essence through the physical deed itself, what else is needed?

Excerpt from Inyonei Toras HaChassidus. The second and third paragraphs are my focus; the first one is for the context. (The Rebbe is discussing the concept of acquiring a physical object through one’s daled amos. For more detail, see here.)

[T]he Yechida manifests itself (not only in the purification of the physical object which is outside the person; but also) in the different levels of the soul itself, the four levels of Nefesh, Ruach, Neshama, and Chaya. Only through them does the Yechida affect the physical object that is outside the person (though the arousal of the Yechida comes solely because of the purification [Birur] of the physical object).

A parallel of this phenomenon can be seen in the power of the Essence of G-d, “whose Existence comes from Himself” [i.e., the Essence of G-d is the only level that does not derive its origins and sustenance from other than itself: it is its own source]. Though this power is specifically expressed through the creation of a physical being¹, nevertheless, eliciting this power of Essence into an independent being comes only through the Light, “the Light is that which mediates between True Being and created being. Thus, through the mediation of the Light, the power of Essence is enabled to bring about the existence of a ‘thing’ from complete and utter nothingness.” (Ma’amar “Yechayenu” 5694, ch. 14 [Sefer HaMa'amorim 5711 p. 391]; see also Iggeres HaKodesh Sect. XX.)

Similarly in the individual’s service: though the manifestation of Essence comes through the fulfillment of the mitzvos of action, nevertheless the manner of drawing the Essence into the performance of mitzvos is only through the inner powers: Intellect and Emotions. (See Likutei Sichos 111, p. 956, that for this reason Love and Fear are called “the paths of G-d”. Examine there further in detail.)

A more detailed explanation here (and in the whole sicha).
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¹ In the above-mentioned sicho, the Rebbe says:

This is also reflected in the concept of (a dwelling [for G‑d] in the lower worlds). It is the entities of this material world which will become a dwelling for His essence, for [only] they possess the feeling that metziuso meiatzmuso (their existence emanates from their selves). This pattern is also relevant with regard to the concept of mesirus nefesh as the sichah continues to explain. See note 35.

[Trans. Note: The term metziuso meiatzmuso refers to G‑d. In contrast to all the entities of the physical and spiritual cosmos, of Him alone can it be said that: “His existence emanates from Himself.” For He never came into existence; He is, was, and will be. Every other entity came into being from absolute nothingness. Hence, its existence is not true. He, by contrast, is the absolute truth of all being.

Within our physical world, however, there is the illusion that metziuso meiatzmuso. In other words, the conscious forms of life in our world feel that they are a genuine form of existence. They do not have the sense that they were created. They may intellectually understand that to be true, but on the level of feeling, their gut sensation is that “I am.”

Nowhere else in the spiritual cosmos — except G‑d’s essence — does such a feeling exist. Every spiritual entity appreciates that G‑d is its source and therefore, feels butel to Him. It is only in our world where we do not possess such feelings and hence feel our own selves so powerfully.]