Why is the Heart Sutra so important in Mahayana Buddhism? So the Heart Sutra is one of the most popular sutra in Mahayana Buddhism, said to be the summary and heart of the Prajna Paramita Sutra. It was popularized by the famous monk Xuanzang. It was said that he chanted this sutra when he was lost in the Taklamakan Desert on the way from China to India.

Apart from its terse nature and convenience for chanting. Why is this Heart Sutra so important to Mahayana Buddhism?

Some people even describe the sutra as a kind of 'crazy wisdom' that seems to intentionally tear down all purposeful cultivation.

http://www.lionsroar.com/the-heart-sutra-will-change-you-forever/

It was even jokingly described as the 'heart attack' sutra.

"Also, there is no truth of suffering, of the cause of suffering, of the cessation of suffering or of the path. There is no wisdom, and there is no attainment whatsoever"

Why would this sutra be helpful or useful - when it basically denies the fundamental teachings of the Buddha on the Four Noble Truth?

Presumably the message of the Heart Sutra was so troubling that Thich Nhat Hanh had to 're-translate' the sutra into something that fit in with his own conception of Buddhism:

http://plumvillage.org/news/thich-nhat-hanh-new-heart-sutra-translation/

What is the purpose of this sutra? Is it just sheerly for the shock factor? Maybe like a Zen Master whacking a student with a stick? I find other Prajna Paramita sutras such as the Diamond Sutra just as troubling.

Note: I do enjoy some of the messages of Mahayana Sutras such as Amitabha Sutra and Universal Gate Chapter of the Lotus Sutra and is from a Chinese Mahayana Chan organization.
Maybe because Clinging/Upadana is such a powerful destructive force even advanced practitioners still have a tough time dealing with. So the non-clinging theme reflected thru insight into Emptiness/Sunyata throughout the sutra would be a great way to remind us about it. That's why in many Mahayana temples, this sutra is usually recited at the end of any session, after other sutras have been recited, sort of a reminder to the practitioner of the Clinging elephant in the room. Anyway, don't worry too much about why it's the most popular or most important. It's a pretty common stock phrase in Mahayana to say this or that sutra is the most popular or most important. You'll see the same thing being said in the Lotus Sutra, the Amitabha Sutras series, etc. Afterall the spirit of the sutra doesn't seems to deviate from the Buddha's teaching about the 3 characteristics inherent in all conditioned dhammas: impermanence/anicca, unsatisfactory/dukkha, and non-self/anatta. And the sutra simply reminds us that it indeed holds true for all "conditioned dhammas", including the Five Aggregates, the Twelve Ayatanas, the Eighteen Dhatus, the Twelve Nidanas, and even the Four Noble Truths!