Having a strong grip is very important for combat sports. I think this is something most combat athletes became aware of over time. Coaches such as Jimmy Pedro focused on gripping very early in his career which in my opinion is a part of his great success throughout his career. Captain crush grippers, IMTUGs, go- really-grip machine,etc. are all tools that have good valuable for building up grip strength but athletes also must focus on grip endurance. Battle ropes, kettlebells, macebells are all great tools to start for building up grip endurance.
Sometimes I see MMA fights where a fighter that works very hard for a guillotine sometimes has his/her arms wear out and they instantly get in trouble especially if they get stood back up. This is a scary place to be. You can have the strongest grip in the world but if your gripping muscles get fatigued and you can't grip, you're in trouble. This not only happens in MMA but I see it happen in Judo and BJJ as well. I think it is important for athletes to have well conditioned gripping muscles so they can recovery quickly from these types of executions.
I earlier shared Martin Rooney's Bar Complex which is also a great way to build up grip endurance. If you have not checked this article out please see my link below.
Video below demonstrates a great way to build up grip endurance. A Towel or part of an old judo, BJJ uniform will also do the trick.