It's used as a proxy for overall strength, as it's very easy to measure in a clinical setting and there is a good amount of data floating about with it.

A bit like BMI, it could be useful for looking at in overall populations even if there are pockets where it doesn't measure overall strength in an accurate manner.

It's not saying if you train your grip strength you will live longer, but _generally_ those that live longer have greater grip strength than those that don't.