You are not alone, it takes some time to understand the system. But it is definitely worth it (well, if you have to build software packages)
For the first OBS is a piece of software. You can install it "at home". However, most people won't do that and for a beginner it will take more like weeks than days to get everything set up.
Luckily the guys at Suse have a public instance at http://build.opensuse.org.
Just register and start using it. Back in the days of MeeGo they also had a public instance, but I'm not aware of any other public ones.
> what is the added value provided by OBS, in contrast to building an RPM on your own machine ?
Actually you can build on your own machine using OBS. It's called localbuild. My last real life use case went like this: A customer uses SLES (Suse's commercial Linux distro with ancient^H^H^Hstable package versions). It's a production machine, I don't want to install compilers and development packages. On my development machine I have OpenSUSE (but I have done the same thing using the Ubuntu OBS client before). Using OBS I can quickly localbuild a package for SLES. The added value of OBS is that behind the scene it sets up a suitable chroot containing the desired target system. I could be any of http://en.opensuse.org/openSUSE:Build_Service_supported_b...
The benefit of building "on the server" is automatic dependency tracking.
If an underlying package has been rebuilt, all depending ones will be rebuilt, too. If you are working on a system with many interdependent packages (or a whole distro) it might be worth setting up your own OBS instance.