When you install using Windows Installer, a copy of changed system info
is placed in that \Installer folder so that if later you want to
uninstall you can restore the original system info using the Add/Remove
function. If you delete that info a future uninstall will not be as
simple (or probably as complete.) It's a judgment call, if space is a
serious problem.
Without those files, you can still remove an installed app's major files
manually, and generally leave some traces of it in places like the
registry, without damaging the system. Registries are accustomed to
accumulating debris and are usually not too vindictive about it.
With major installs like service packs, the restore files can be very
large. They can also be very important, in a case where a service pack
must be removed for technical reasons. There is no manual way to remove
an installed service pack - system re-installation or the equivalent is
the only practical option. However, I believe service pack restore files
are saved in a different folder (\ServicePackFiles), not \Installer. I'm
not entirely sure about this.
