With this feature on, it lets you adjust both the program volume, and alerts volume with the volume rocker. This can get to be annoying though, because the rocker will do different things depending on what screen you happen to be on. I've accidentally had blasting volume when I wanted silence because I lowered the volume, but it was actually the ringer volume, while the program volume was still cranked.
With this feature off, you adjust your alerts volume in the settings menu, and it stays fixed with the volume rocker having no affect on it. Then the volume rocker is always adjusting your program volume no matter what.
A little wonky in my opinion, though I'm not sure how else they could implement it. Other than having one master volume for everything, which I'm surprised they don't have. Maybe in an update.