Back in the day, batteries died. If your Sonic 3 save corrupted, you lost your Super Emeralds. But with the lock-on technology, the game treats the combined cart as a single, massive Sonic 3 & Knuckles experience.
You get a proper save system for the entire 14-zone marathon. You start at Mushroom Hill, go through Flying Battery, and eventually crash the Death Egg. The flow is seamless. No "The End" screen in the middle. Just pure, uninterrupted velocity. sonicknuckleswsonic3bin extra quality
To achieve sonicknuckleswsonic3bin extra quality, you cannot simply download the first file you find on a sketchy pop-up site. You need the "No-Intro" standard. Back in the day, batteries died