pkg_comp is a small utility designed to build packages inside a
clean chroot tree.

Some ideas about what to use it for (taken from manpage):
* Build packages for other system versions. For example, build
  packages for NetBSD 1.5 while you are running NetBSD current.
* Build packages using different options than your current system
  like changing the threading library, COPTS, placement of
  configuration files, etc.
* Debug the build process of a package, checking if buildlinks
  work properly.
* Avoid autoconf's side effects by keeping a separate chroot for
  each project, like one for GNOME2 and another one for KDE3.
* Schedule builds of package sets for several different machines.
