Due to licensing, copyright, and maybe censorship issues, not all apps are available worldwide. This means that while some users can access certain apps, others living in another part of the world can't. This has led to smartphone users coming up with unofficial ways of circumventing these restrictions.
However the good news is that for Android users, it looks like Google will be easing up on those restrictions and allowing some Android users to shop in a different country's Play Store. This means that for some, they will be able to make purchases in a Play Store different from the one that they are using, although note that this comes with a caveat.