If you attach Sonic 2 to the Sonic & Knuckles lock-on this unlocks "Knuckles the Echidna in Sonic the Hedgehog 2", wherein the player can play the game using Knuckles and his abilities. 
Using these special moves, Knuckles can explore areas inaccessible with Sonic or Tails, where secrets such as extra monitors have been added. 
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GIF