For distributor of the app (app stores) - they can take a percentage of sales if the app is monetized.

For users of the app - As you mentioned, speed is an important factor. But we can also trust that the distributor has reviewed the app and confirmed that it's safe to use (theoretically at least).

One more point I would add - app stores provide a singular place for me to search, compare, and discover new apps that might be valuable to me.