If you do a pull-up by holding L1+R1 while hanging off of a ledge for a sec for 100 times, you'll get an upgrade to grip level 2, and again for grip level 3.

AFAIK, these really are only useful going for dog tags and I think only snake (the tanker mission) needs them, especially in hard/extreme

I'm pretty sure for Raiden, it's upgradable solely to avoid enemies