Fat grips are a worthy addition to a training program.
Fat grips are not essential, and aren't a "miracle". Sorry to say.
Nobody has some out of control strength, but weak hands and grip that negate it .
The only ppl who do, are people who aren't really that strong (ie someone who just benched a lot and got real good at the bench).
When you train hard and right, your grip strenght and hand strength will invariably go up, in unison, as you progress.
When you lift and throw around heavier weight, your hands and grip get trained.
Surely, supremely strong hands and wrists are a plus -- especially if you are a fighter, for injury prevention purposes. But you can train your hands, grip, and wrists with a couple mini workouts throughout the week, and get great results.
You don't need a fat bar. I like fat bars, though, and probably will buy one soon. I feel I can lift more weight with them, as it increases my body-weight connection.
Same reason I train (and run) in a pair of Adidas Adipiures.