Rolling releases are popular because people got sick of waiting two years to upgrade their distro and get the new version of some Linux app, because one version of a distro keeps the same old version of the Linux apps forever (in the stable tree). unstable and testing branches have newer releases, but as the name implies, it breaks quite a bit.

So rolling releases are like an unstable/testing branch, with more effort put into keeping it from breaking. So you get new software all the time. The downside is, you also don't get to opt-out of an upgrade, which can be pretty painful when the upgrade breaks something you're used to.