A number of things. I'm comparing it to the website, though I think a lot of this holds true to the app as well.
1. It loads content significantly more quickly than New Reddit (Reddit has done a lot to improve on this, but for a long time, the New Reddit™ was horrifically slow after scrolling through a few posts.)
2. It doesn't support any of the social media functions that the New Reddit provides, which, for me, is perfect because that's not what I browse Reddit for. (You can turn all of that off in the New Reddit, though.)
3. Way less busy than New Reddit, even if you use "Classic" view inside of "Card" view.
4. You can disable infinite scroll in Apollo, whereas you can't in New Reddit (and it's not even supported in Old Reddit unless you use RES)
5. The Apollo developer was insanely responsive to user feedback and was really, really good about incorporating as much as he could into the app (an iPad-native experience being the biggest exception).
1. It loads content significantly more quickly than New Reddit (Reddit has done a lot to improve on this, but for a long time, the New Reddit™ was horrifically slow after scrolling through a few posts.)
2. It doesn't support any of the social media functions that the New Reddit provides, which, for me, is perfect because that's not what I browse Reddit for. (You can turn all of that off in the New Reddit, though.)
3. Way less busy than New Reddit, even if you use "Classic" view inside of "Card" view.
4. You can disable infinite scroll in Apollo, whereas you can't in New Reddit (and it's not even supported in Old Reddit unless you use RES)
5. The Apollo developer was insanely responsive to user feedback and was really, really good about incorporating as much as he could into the app (an iPad-native experience being the biggest exception).