[Cicero22]

another possibility is to increase traffic, which I guess also increases ad revenue. It's a lot easier to tap an icon on your home screen than it is to open a browser and type in reddit and whichever subreddit you want to browse. The less friction there is, the more likely you are to be a daily user, driving their revenue.

[joegahona]

This is my guess — phone-screen real estate and the potential for push notifications.