Grip Trainers might not seem that useful unless you’re trying to build Popeye-esque forearms or develop a crushing handshake. It's reasonable to ask if there i any real purpose for grip trainers? Sure – if you fall into one of these categories...
Bodybuilder: As you start pushing your body to its limits in terms of weight and stamina, failure will come at the body’s weakest points. Sure, your biceps may be capable of curling 90+ pounds, but if your hands aren’t strong enough to grip it, you won’t get in any worthwhile reps. Sure, you could hit the machines, but then you lose out on the stabilization exercise that comes with free weights.
Beginner or expert, your grip in rock climbing can literally be the difference between life and death (or at least challenging fun and serious injury). Grip trainers can make sure your hands are capable of holding your entire body weight if need be whether for a split second or several moments.
Mixed Martial Artist:
Holds and throws like those in jiu-jitsu and wrestling are infinitely more effective when hand strength comes into play. From thumbs to pinches to full-hand grasping, the better grip you can get on your opponent or yourself (for submission holds and the like) the more successful you can expect to be.
Fact - criminals and inmates aren’t pushovers. The ability to hold them at bay is crucial in the life of a police officer or corrections officer. Whether holding back a riot or disarming a suspect, the upper hand is usually the stronger one.