It gives users easier access and a better experience, so they're more likely to return and to spend longer. The ultimate goal is to become a "default time waster" for as many people as possible, who open the app whenever they're bored.
There's no "better experience" in an app, unless it relies on native features like low-level high performance graphics, accelerometer/sensors, and other physical phone attributes not yet exposed to browsers.
And Sync and AlienBlue if they're still around. It blows my mind that one dev is able to push out such a high quality product where the official app feels amateurish by comparison. It really highlights the difference between development that serves Reddit vs development that serves the user.
It's a lot easier to run browser based ad blockers now than it used to be. Mobile Safari now allows them and installing an adblock extension in Firefox for Android is as easy as on desktop.