Lupin seed has been used in animal feed for a long time - cattle,poultry, pigs etc, - and now the 'grit' is being used as an additive in the human food chain.
All about textural analysis of the product - the mouthfeel when you eat, etc. The crunch of the snack is increased by the grit as the steamed potato flour that makes the base for the snacks has little texture alone, so to make it palatable, they add the other ingredients - and spend years researching the processes and shed loads of money finding out what is the optimum amount of carrier oil to pass off to us to hold the flavours on the products, what chemicals can be added to prolong shelf life etc etc - all tasty stuff. . . .
Almos as a side effect is the fact that lupin as above has good nutirion value - but I'd argue this only really adds to a makers claim that a manufactured and 'constructed' product has value when eaten.