The Rolling Release Development Model
It would seem that some confusion has arisen due to the regular (and somewhat rapid) occurrence of new Manjaro releases. This confusion has been compounded still further by the use of version numbers (for example: 0.8.1, 0.8.2 and so on), a custom normally associated with the Standard Release Development Model, where a brand-new release of an operating system must be re-installed over an old version in order to access any new features provided.
However, Manjaro uses a Rolling Release Development Model, whereby rather than being replaced, the same core system will instead be continually updated and upgraded. As such it is not, nor will it ever be, necessary to re-install a later release of Manjaro in order to enjoy the very latest and most up-to-date system possible. By virtue of keeping an existing installation updated, it is already the latest release.
An overview of the three repositories that Manjaro uses to manage its packages can be found here: 
Why are new versions of Manjaro being released?
These releases are more accurately new snapshots of the Manjaro system. Just like photographs, these snapshots are in essence images that portray the Manjaro system at a particular point in its development. As such, one purpose they serve is to help the developers to both chart and evaluate the continued development of the Manjaro system, as it is of course easier to pick up on the finer details from a single image than from a moving picture.
Furthermore, due to being in a beta stage of development, Manjaro is evolving at a very rapid pace. Existing users may themselves have noticed how frequently new software updates and upgrades have become available, and how much the system has changed in only a few months. As a consequence, a snapshot of the Manjaro system --again rather like a photograph-- will very quickly become out of date. New snapshots are therefore released in order to make life easier for new users, who may otherwise be faced with the prospect of downloading huge volumes of new software packages to update and upgrade their newly installed systems from an older ISO.
Where these snapshots most obviously differ is in the choice of pre-installed software applications provided, as well as other smaller tweaks such as the default theme(s) used; however, the core Manjaro system running underneath is the same and any applications added or removed from a particular snapshot can also be added or removed from an existing installation. And, of course, users will tweak and customise their own systems in accordance with their own personal preferences and tastes, anyway. Again, irrespective of whatever release of Manjaro has been installed, provided it is kept up to date, then that will be the latest version available.
Why are new releases given version numbers?
Quite simply, version numbers have been assigned to snapshots of Manjaro by the developers to help them clearly mark each stage of its development. For example, amongst some other planned changes, once development has progressed to the stage where a user-friendly graphical interface has replaced the current text-based installer, then those snapshots will be assigned 0.9x version numbers.
Users may also have noticed that version numbers currently all begin with a zero. This is used to indicate to the public that Manjaro is currently still in a beta stage of testing and development, as it has not yet reached an officially stable first release. However, we do appreciate that the use of version numbers in this manner has indeed been the source of some confusion, and it is consequently likely that upon the first stable release of Manjaro, they will be replaced with the dates that the snapshots were taken.