It had extra features, such as a cartridge expansion ... It featured a library of games developed for the Game Gear and direct ports of Sega Master System games, such as Sonic the Hedgehog.
Anyone that grew up in the early 90's and had a Sega ... game (which I still believe 32 years later is one of the best games of all time) or Sonic 3, you could get the Knuckles expansion cartridge ...