First I'm not expert, this is my personal opinion based on what I have read, and things that I been told by people that study the Dharma, the field in enormous and require lots of study, reader is advice that I could be wrong.I will add a short list or my resources at the end.
I believe the reason of its importance is because it includes the complete teachings of the Vajrayana Buddhisms in it. Emptiness and Wisdom.
As well give a concrete explanation based on logic and prove that everything is empty of the essential nature.
I pass to explain why I thing this, but first a little bit of context
Where the Heart Sutra comes from?
The short answer is that it comes from the Buda, or at least the Buddha is giving the teaching with out giving it. (Emptiness)
Since, start with the phrase “ Thus i heard on one occasion...”. Is believe that confirm authenticity as was used to prove this since earlier text.
Also there is Tibetan lineages with proven unbroken lineage that have the Heart Sutra as teaching since their origins (some old as 1000 years old, Sakya linage).
Beside commonly believed to be an oral tradition from the Buddhas times, hard copies have been discovered, some old as around 1400 year in what we known today as China. The work stand by itself, as its use logic for the demonstration of the of emptiness, although we still need faith to believe that comes from the Buddha's time.
To understand better, probably is necessary to have certain understanding of the “frame of work”, which is the human body. The definition of a human body was a bit different as is today, and the text refers to this when mention:
“...who wishes to practice the profound perfection of discriminating insight should discern in this way: They should correctly view those five aggregates also as empty of essential nature.”
The five aggregates are: Five senses Organs, Feelings, Perception, Conditioning factors, consciousness.
They are a “part” of the enumeration of the phenomena and
Enumeration of phenomena is kind of the definition of the human experience, or a human, but keep in mind that this is a way to break down what can not be broken down.
|---Eye and the object to see
|---Ear and the object to hear
|---Body, Object of touch
|---It also considers the imperceptible
The reason that Avalokiteshvara (Chanrezi) is a figure in the Heart Sutra is because while practicing compassion, we see others as our-self or our-self as others and that is very helpful when we are trying to understand emptiness.
Also emptiness is represented as a illusory lady in the same text she is called prajnaparamita.
Please forgive any typo and this is just my personal opinion. Gracias!
English version by Leo M. Pruden
Asian Humanity press
A praice of the twelve exemplary deeds of Buddha-The Heart Sutra-A non- sectarian prayer.
Sakya Monastery of Tibetan Buddhism
English Translation Cyrus Stearns
edited by Jeffrey Schoening
Sakya Monastery 2000