Lots of people use these to improve stamina for guitar playing, so do I. I did find that it helps with anything where you need hand and finger strength, such as sailing, paddling, climbing, etc. I even gave one to my grandson to help with his wrestling. Nice and small, it allows attention to individual fingers, making it better than the spring grip style.
I use this to train my fingers when I can't go to the gym. It's lightweight, you can take it anywhere in your purse and you can grab the whole thing in your palm so you don't get weird looks in your direction. And it works. Also good to warm up your fingers before hitting the rock.
I use it to strengthen my guitar playing hand. I have found it definitely helps strengthen my fretboard hand, making playing more fluent and requiring less effort. It's small enough to comfortably fit in your pocket, so you can take it anywhere. If I lost my Gripmaster, I'd be out buying a replacement the next day.