An example is better than words.
First thanks to shironeko to contribute.
You can find its own OUR
here
Do you want emacs without systemd already built for you?
Just download the package
here.

Extracts the zip archive somewhere and install it by pacman -U.
You want to see the build logs of the emacs package?
see
here
You want to retrieve the PKGBUILD for a previous version?
see
here
So, everyone can maintain its own package and share its binary easily with other user.
Furthermore, The package is build on the same environment than any official Obarun packages.
You want to be notificied when the package change?
Just use the notification feature of the gitlab instance