The standout feature is the physical , where you could plug the Sonic 3 cartridge directly into the top of the Sonic & Knuckles cartridge.
Unlike the standalone Sonic & Knuckles, the combined game includes a save system , which is essential given the game’s significant length (about 4 hours for a full run). Presentation and Sound
— An essential classic that perfected the Sonic formula before the series transitioned to 3D. I.R. fan project? Sonic & Knuckles + Sonic the Hedgehog 3 & Knuckles Review