Pkgbuild-watch alerts you to upstream activity. Between AUR and TU duties, I have about 130 packages to look after and this checks all of them in 10 seconds.
Maintainers. If you are a Dev, TU, or anyone with AUR packages, pkgbuild-watch will let you get updates out to your users faster.
Rebuilders. If you maintain your own source ABS tree, pkgbuild-watch will tell you when any Arch package gets an update.
Flaggers. Somewhere a maintainer is sleeping on the job. Find reasons to pester them with ease, though you'll still have to click the flag-out-of-date button yourself.
Path is searched for PKGBUILDs. If you have multiple paths (say for both AUR and ABS), create a dummy directory with symlinks to each.
The PKGBUILDs are all sourced (be careful with that!) and the
$url is watched for changes. If the homepage url is not satisfactory, set the
$_watch variable to the download page instead.
There are two magic values for
$_watch. First is
"none", useful for VCS packages. The second is
"package". This uses the
$pkgname to generate a link to
http://www.archlinux.org/packages/, so that you can be notified when the Arch package is updated.
There may be some debate as to whether the
$_watch variable should exist in the pkgbuild at all. I think it should, because it benefits everyone and seems the most sensible place to do this sort of thing. I would like to hope that someday this feature could become an official part of makepkg (losing the underscore in the process). We have Namcap to automate post-packaging, and pkgbuild-watch fills the pre-packaging void.