Well...
The MaxDrive is a USB Flash Drive - you plug it into the PS2's USB port and can download (or rather upload, since the PS2 memory card is 8Mb and the MaxDrive is 32Mb) your gamesaves to the MaxDrive. You can then bung the files onto your computer. Using the .max/.xps file convertor, you can turn your files into XPort files, which mk's GT3gEdit programme can read (not to mention the fact you can back up your memory cards to the computer, to prevent drunken/younger brother game save erasure).
GT3gEdit is a very, very nice programme, which allows you to create hybrids pretty much at will. There's a very easy interface which allows you to see all of the cars in your garage and what parts/settings are applied to them. You can change any part of any car to any part of any other car in a matter of 3 clicks of the mouse and create entire garages full of hybrids in the time it takes you to type in a stage 4 turbo code on one of the cheat cartridges. See my sig to see just how easy it is.
As for why it is different to OTHER USB flash drives... It comes bundled with a disc to allow it to operate on the PS2 and is £10.