Whdload is for installing old games on HD, that take over the hardware from the OS or poke the hardware directly. It allows you to start them from the OS and exit returning to the OS.
It also fixes them so that they work on slightly different hardware (updated CPU, gfx chip etc.).
For programs you might need to use a degrader, though I can't think of any program that doesn't have an updated/better equivalent on the A1200.
