A couple more observations on wrist grabs. Firstly it is easy, particularly if you are male, to dismiss the scenario where some comes up to you grabs your wrist and remains static. Yes as a man it is highly unlikely that another man will do that. I have however, on more than one occasion, witnessed a man and a woman arguing in (or outside) a pub on a Saturday night. Usually it’s boyfriend/girlfriend having an argument. The girlfriend wants to either keep her drunken/angry boyfriend away from her and so puts her hands up to keep him away, and he garbs her wrists so she can’t. Or, she turns to walk away and he grabs hold of her arm and won’t let her. Alternatively the drunken/angry girlfriend is slapping/pushing the boyfriend and so he grabs her arms to stop her.
Secondly wrists grabs do indeed have a have a very practical real world fight/self defence application. If you were to strike pre-emptively (for the purposes of self defence), a natural reaction of your target would be to draw their hands up to protect their head. These hands are then preventing you from continued striking and (at least to the head where the knock out blow is) however if you seize the arms and strip it out of the way, you can continue striking.
Taking this to the next logical step, if someone started throwing punches at you and you manged to get your hands up to protect your head, your attacker may do the same to you - grab your arm and strip it out of the way to facilitate further strikes, so then then there is a very real need to address to problem of your arms being grabbed.
This expalins it a lot better:-