It’s marketing, it makes people with an app feel special. Sales will increase more than discounting to everybody.
That’s kind of an “older generation” take, to be fair (soz!). For most people, apps are just part of life in order to access stuff (banking, commuting, post, email, air-travel). More for ease than to “feel special”. If you can have something confined to an app on your phone, you don’t need to arse about with bits of paper, cards, keys, weird banky-type tokens, etc.
They appeal especially younger people with in-app games such as spin-a wheel to get a discount or free item, or event-led ones like shooting a ball into a goal to get a free item.
People like surprises and winning things (even if they have zero actual chance of losing).