HD Advance is a cloned version of the HD Loader software for the PlayStation 2, which allow users to play games installed to the optional hard drive peripheral available for the PS2.
HD Advance is identical in functionality to many patched versions of HD Loader freely available online (that include DVD-9 support and 48-bit hard drive addressing) - only with a different theme and some changed text strings.
The only benefit of HD Advance is that it is distributed on a pressed disk, giving a patched version of HD Loader that can run on a console without a modchip installed, and also without the need of the PS2 Independence Exploit or any other software modification.