I've never used _service. My way of getting the source archive is to use rpmdev-spectool -g path/to/foo.spec. Is there any benefit of using _service over that?

https://openbuildservice.org/help/manuals/obs-user-guide/cha.obs.source_service.html

In this case, it's useful to have an archive with the right name instead of having to define a _version.