I've kept all of mine so far. I don't know if they will ever have a "use", but I kept all of the proof of purchase from my Star Wars toys from the 70's and 80's. It just seemed like the right thing to do.

They are a physical reminder of the money you've spent on that product.

The geniuses at FFG put it in there as a cardboard punch out because we all love punching out cardboard pieces with our thumbs. Their sales/market researchers and psychologists have also rigorously conducted surveys and experiments of the positive cognitive effects of punching out cardboard pieces, which led them to the conclusion that by ever so slightly reminding you that your proof of purchase is something fun to punch out after the fact, you are more likely to do it again with less or without hesitation. This subliminal form of marketing has proven successful in such addictive ways that we buy up X-Wing Miniatures like the plastic crack that it is.

All of this is because we have been psychologically conditioned to enjoy punching out the proof of purchase pieces after we purchase our minis.

FFG do this with all of their games, as far as I know they can't be used for anything.