The Heart Sutra is of utmost importance in Mahayana because it declares the key realization that makes Mahayana distinct from a superficial "Sutra-Yana":
That in the ultimate sense all concepts are arbitrary, including even the concepts comprising Buddha-Dharma, that ontological reality behind concepts is by itself free from any discriminations, that freedom of mind from the clashes induced by conceptual discriminations is the very freedom from suffering that Buddha taught.
Heart Sutra is the Mahayana's Declaration of Independence in that it proclaims the heart of the teaching, the transcendental wisdom, the wisdom that goes to the end of the realm of thought, the limits of conceptualization.
It's not written for a shocking effect. The reason it emphasizes emptiness of even the most fundamental Buddhist concepts is to drive home the idea that everything is just a mental overlay, and if that's removed then behind it we find no dirt and no need for cleansing, no entrapment and no need for liberation. Our only problem is entanglement in the illusion that these things are real and important.
All past, present, and future Buddhas attain Buddhahood through their realization of this freedom from conceptual discrimination. This realization is not something external to the Canon but is the real meaning of the liberation as taught by the Buddha in the Canon. Every progression of Jhana states described in the Canon culminates in the very same realization. The parable of the raft, the concept of tathata, the numerous Canonical descriptions of Nibbana - all refer to the same realization.
This proclamation is the heart of Mahayana and that's what makes this Sutra the crown jewel of all the teaching.