Bouncebacks and inertia scrolling help the user experience because they're closer to real world behaviour. On a subconscious level, people more easily interact with something that has a little weight and closer to real physics to it.
It solves some real world problems, but is it really closer to real world behavior? If I for example flip through a photo album and reach the last page, it wont bounce back. There are other cues to that, such as the thickness of the page or if I attempt to flip the last page I will instead see the back cover etc.