I don’t work for Spotify but I am a mobile app dev and while this could easily just be trying to funnel people into the app it could also be focused on preventing bots and fraud.

Mobile phones offer significantly better tamper resistance compared to laptops and PCs. If someone gives a rating from your app on a phone there are several different forms of attestation available to be sure it’s not a bot or fake account, none of which work on PC.

And if you’re trying to combat the rampant fraud that happens around review manipulation, that is one solution.