With the app, you can not only send notifications, but you also have the app icon which reminds people of the apps existence. Even little things like "badging" (when the app icon shows a dot or number to indicate new notifications) and even that can have a noticeable impact on retention (we're all programmed to click into anything with a notification).
Also, they can recommend better content for you. I don't know if this is still possible and whether reddit does it, but an app used to be able to get the list of other apps installed on the phone, and many companies used that as an input into their recommendation systems (along with your location, etc).
