A maintainer needs two things from an upstream - latest and greatest version of the software that is considered by the developer to be ready for public consumption (this goes into unstable as a new software version) and one-liner patches fixing security bugs in the version of the software that made it into the last stable release.

It is also nice to know in advance that a particular upcoming version will be a rewrite, so there might be more bugs and you might want the previous version to go into the stable release or that this is a special version that author will commit to supporting with security patches for a longer time.