Over 20 Gameboy games have special Super Gameboy functionality that allowed for multiplayer gaming by only needing one cartridge. Many were exclusive to Japan.
Normally, 2 Gameboys, 2 copies of a compatible game, and 1 link cable are needed for two-player multiplayer.
However, all the games featured in this video support 2-4 players by only needing a single Gameboy cartridge and a Super Gameboy used in a Super Nintendo. Obviously extra controllers are needed too. But it beats going through the chore of tethering systems with a link cable. A few games actually support the SNES MultiTap accessory to allow for 4 player simultaneous play!
And as an FYI, this is only available on the Super Gameboy. The Gamecube’s Gameboy Player (GBP) does not contain Super Gameboy functionality.