Not so much a tech feature, but one that goes hand-in-hand with one. Merchandise is a great way to increase brand recognition and the QR codes from Spotify.
It’s possible that a random person will see these codes on a bag or shirt, recognize that symbol and look up what the codes are. It could make non-users more interested in the Spotify brand.
Yes, sure it might be a little strange for people to go around “taking pictures” of people’s shirts and bags. Let’s hope they ask first or just think “Hey, that’s a pretty cool logo and shirt. I really want one.”