In Muay Thai, when you’re opponent tries to throw a knee, they hold the back of your neck and pull you.
A knee to the stomach or plexus is really powerful, so you have to act fast.
The best defense in those case is to hold the back of their neck in return, pull them closer so that they cannot strike, and try to throw a knee first.
It ends up with a play of the two opponents holding each other and trying to throw a knee while closing the distance.
And most likely ends up in the dead lock or “hugging” that we see in match.

I’m sure there are other techniques that involve this holding in MMA. Forbidding it would remove those techniques from the game.