Essentially you should only need it if the normal add remove programs uninstall does work.
Or if there are complications with and update to a much newer version, like avast5 to avast6, but that is normally not an issue.
After such an update if components aren't working or not working as expected, it may be best to do a clean install. Uninstall, reboot, run avast uninstall utility for safe mode, once for the old version, then for the one you just installed, reboot and install the latest version.