Some people would like to use the new features available in the next release. Others want to help contribute to Emacs by testing the development sources and maybe even report EmacsBugs. Others are just interested in following Emacs’s development. Some other just like “living on the edge”.
For information on how to get them directly, see:
See BuildingEmacs for how to build Emacs from the source package.
See EmacsDevelopmentFeatures to find out what the new features are.
Binary packages built from Bazaar snapshots are available for various systems: