The cartridge slot is for the battery back-up ram cart, or some games (the capcom dungeons & dragons games, several snk fighters) came with ram carts because they required extra memory to run properly. The action replay, which I'm told can make your saturn region free, also uses that slot.

Heres some top notch saturn games:
Pulirula
Gradius deluxe pack
Salamander deluxe pack
Nights into dreams
Guardian heroes
Panzer dragoon 1 and 2 (zwei)
Astal
Shining wisdom (buy an english copy)
Shining the holy ark (english)
Shinig Force 3 (english)
Panzer dragoon saga (good luck with that)
Saturn bomberman (as previously mentioned)
Die hard arcade
Elevator action returns
Night striker
Daytona usa (i like both versions. Blasphemy, I know)
Legend of oasis (english)
Any parodius game (sexy parodius is my favorite)
Twinbee deluxe pack
Dracula x: nocturne in the moonlight (castlevania symphony of the night with extra areas and character selection from game start. Also with horrible load times.)
Dark savior (english, absolute MUST play; spiritual sequel to land stalker. Brilliant game.)

I'm sure there are more, but these are some of my favorites. Despite what people will tell you, the saturn is a fantastic system. It's great for 2d games, and the 3d games got much better towards the end, when sega released a better programming library.