This lets the user define that only the master branch should be tagged.
Other branches or pull requests will compute a new version number (bumped as usual). But the version will be marked as a pre-release thanks to the pre-release postfix. So, instead of computing 1.2.3, the script will compute 1.2.3-1a2b3c4 and the next revision of the Pull Request (or feature branch build) will compute a similar version 1.2.3-2b3c4d5 (since no tag was put and supposedly master did not evolve in the meantime).