Distributions are pre-configured, and tested, versions of AROS. They contain a lot of useful 3rd party applications that don't come with the default AROS.org binaries, and will be of great interest to users.
They often may not have the latest core system components, but should offer better stability and user-friendliness than the nightly builds. If you are a user interested in checking what AROS has to offer, you are highly recommended to use distributions, to get the most complete AROS experience.