In reality people collect souvenirs from all sorts of things. Why should game characters be any different?
You can have people collect enemy weapons, or teeth, or whatever as mementos. Maybe some place becomes significant because of the adventures had there, and some of the heroes want to take something from the place as a keepsake. Some of these things might be precious in monetary terms, but perhaps someone doesn't want to sell them to turn into cash. Rather, you can hang on to looted jewels or trinkets and they end up serving a very different purpose. They can become character development and objects of reminiscence and nostalgia, rather than wealth.
Commentary by memnarch (who has not seen the movie)
Hah! Finn and Rose are going to be delivery folks! And using something that Finn currently has is a pretty clever way to get him out of the spaceship chase. And covers up the fact that Finn had a bunch of things he was trying to get rid of as well quite handily.
I think that serial number is a Pulp Fiction reference, but I'm not sure if there's any other significance here. Maybe there's something similar in the dialogue I won't get? I'm really enjoying just how into this whole thing Sally is though after the initial disbelief though!
Man, Kanata looks way too excited about all of whatever they're talking about. It fits so well in the comic here, but I can't think why he'd be making a face like that in the movie. Especially since that shoot-out a few comics ago looks to be real. Hmmm, maybe things are reversed? Kanata brightly says hello, everyone talks a bit, then a shoot-out starts happening? I suppose that could work, though we'd still need a reason for the appearance here and the lightsaber was already given away in the last movie.
Poe: We need something Darth Kanata can order, that they’ll have to dispatch you to deliver. Whaddya got?
Finn: I had to abandon most of it. All I have is a First Order comlink, code book—
Darth Kanata: I’m on their website. They don’t just sell those to anyone, you know.
Finn: —and my Peace Moon cafeteria card.
Darth Kanata: Ooh! Surplus stocks of those are being touted as collectible souvenirs. There's a field for requesting a specific number. What’s the serial number on yours?
Finn: Um. EZKL-2517.
Darth Kanata: My lucky number! I’ll order that one. Delivery address... Canto Bight.
Finn: This is never—
GM: Finn, your comlink has a message ordering you to deliver your cafeteria card to Canto Bight.
Darth Kanata: I paid extra for express delivery!