It’s intended to help you use your iPhone one-handed, which I think was a reasonably good idea on Apple’s part; however, here’s what my personal experience with it has been.
Actually, I think the “times I meant to use it” may even be a bit exaggerated there.
So if you’re like me and just keep triggering your screen to move without meaning to, here’s how you turn Reachability off entirely. First you’ll go to Settings> General, and then you’ll pick “Accessibility.”
Scroll down to the bottom, and you’ll see the Reachability toggle.
If you turn that off, your screen won’t slide around at all anymore, and you’ll become a much more fulfilled and happy person. OK, that’s probably not true, but you won’t have to worry about inappropriately moving iPhone displays, at least.