As a self-proclaimed SEGA fanatic, I think I can say the point of it.
The Sega CD add-on was basically a cash-in attempt to get into the CD market before it was taken over by other companies such as Nintendo, following the release of the Turbografx CD for the Turbografx 16. They had high hopes for its greater storage capacity and audio potential, but, honestly, it had its flaws, and was advertised for the digital video potential it never really had or could pull off. Many factors decreased sales, such as the Night Trap case, which, looking back, was absurd, and the fact that the Playstation hit a few years later. That being said, it has quite a cult following, with its exclusives such as the cyberpunk adventure game Snatcher.
Then there was the 32X, which stuck in the top of the console, and connected to an expansion port in the rear of the unit. It had its own cartridge library, but could also use the Sega CD with 5-10 32X CD games, which came on a disc, and utilized both the SEGA CD and an attached 32X to play back better quality games. However, the 32X flopped, as nobody wanted to stick the money into the thirty-ish cartridge games made for it. They really didn't add enough, and fade3d out of necessity.
They do enhance the Genesis. There were many games which required these add-ons, but the Genesis always sold more units, so more games were made. However, these fixed basic flaws with the system which hurt it as it aged, and attempted to prolong the life until the flop that was the Saturn came out. They considered releasing a new system early on, but the cost for what was basically an enhanced Genesis made it pointless to do. So, they released the consoles as additions to the original machine, and had it built to be able to play better games.
