What was the cartridge slot in the Sega Saturn useful for?

There were many potential uses mentioned by Sega about the cartridge slot  The ones that we know about include the memory cart for game saves,

A new method for running code on SEGA Saturn off an SD card has been developed — and it works via the Player 2 controller port

sega slot machine It does this by taking advantage of an underused add-on slot: the Video CD cartridge port, hidden in the back of the Sega Saturn by the battery

playtech slot download Saturn Homebrew Cartridges ; USB dev cart, USB port, @300KBs, No extra RAM ; Saturn Gamer's Cartridge #new#, SD card, @40KBs #new#