Mahakaal is the Ultimate Ruler, Shiva is known as Mahakaal because he is the one who can control time and death (Kaal), he is beyond the concept of time.
So there is a popular story of Rishi Markandeya. His life span was supposed to be only 16 years. However, despite being a very young boy, Markandeya had great command over the Vedic mantras and he had mastered the Mahamrityunjay Mantra.
As a result, When Yama (Yamraj) the God of death came to take him. Bhagwan Shiva appeared, stopped Yamraj and gave the boon to Markandeya, Rishi Markandeya is believed to be the 8th Chiranjivi because he is blessed by Shankar Bhagwan himself.
So the significance of Shiva as Mahakaal is that he is the one who change the destiny of a person written by Brahma. So basically Mahakaal has that authority to edit Brahma’s design.
Mahakaal represents the eternal consciousness, he is the most intense, fierce yet the most stable, unmoving.
One more thing about Mahakaal is that he is Bhairava swaroop. There multiple versions of Shiva’s Bhairava swaroop, among them, Mahakaal is the topmost. He is described as Anant Koti Brahmand Nayak Shri Rajadhiraj Mahakaal. Like Kaal Bhairava is known as the Kshetrapala, the guard and then there are many more manifestations of Bhairava. So Bhairava is the one who makes you fearless, when you surrender to Mahakaal, you become truly fearless and you love truth.
Mahakaal Bhairava is that Ultimate Creator, Protector, Destroyer and Transformer. So this is the significance of Mahakaal Baba.
Jai Mahakaal 🙏