It’s just there to digitally preserve ‘Lock-On-Technology’ for the Genesis.
There’s no real benefit to normal end-users but I can think of two uses for really dedicated gamers.
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You can simulate the experience of entering codes one character at a time into a Game Genie menu and then playing Sonic the Hedgehog with infinite rings.
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This is a passage I got from Sega Retro regarding connecting Sonic & Knuckles with a non-Sonic game.
When a non-Sonic game is plugged in, the result is a single stage of Blue Sphere, produced randomly from information in the inserted cartridge’s ROM header, along with the stage’s ‘code’ for use in the full Blue Sphere. If Locked-On with a game larger than 2 Megabytes, it will boot to Sonic & Knuckles on real hardware, as the Sonic & Knuckles cart’s mapper will map the last 2MB of the locked-on ROM instead of the first 2MB. Locking-On with a game that uses a battery pack erases saved games. Most games made after Sonic & Knuckles was released are larger than 2MB and thus will not work with its lock-on feature. There are only a few games made before Sonic & Knuckles that do not work, such as Phantasy Star IV: The End of the Millennium & Super Street Fighter II: The New Challengers.
You can play random Blue Sphere levels, if that’s your thing.