Grip scripts are self contained replicating UObjects that can be instanced inline to a grippable object and used to modify / track the grip behavior of it.
They are like “logic capsules” or “grip components” that can inject additional logic into the gripping chain or even entirely override it in places.
Being an array in the grippable object they will be called in order when querying them, this lets you utilize grip by stacking them to effect scripts down the line. When deactivated a grip script will be skipped from processing until re-enabled.
While a little dated (as additional features have been added since then), this video goes over the blueprint form of grip scripts:
The MeleeTools, GunTools, LerpToHand, Physics Prop grip scripts are all examples of advanced interactions being handled inside of a grip script so that they can be modular and added to any object without sharing a base class.