On the Nintendo GameCube what was memory card slot b for?

Memory Card Slot B, as the name suggests, is the secondary memory card slot. While it wasn't heavily used (as most games typically look for their save file on the memory card in Slot A), some game did support it.

One game that fully utilized Slot B was Animal Crossing. You travel from your town (Slot A) to your visiting town (Slot B) to check things out. If you had more than 2 memory cards, you could actually chain visits by swapping out the currently unused memory card (so once your in the town from Slot B, you can swap the card in Slot A to prepare for the next visit in the change... when once you in the town from Slot A, you can swap the card in Slot B... and so on, and so on UNTIL wish to return home. At that point, you place your original memory card into the open slot for the trip home).

Savegame management is typically a key reason for Slot B, so you can move & copy save files between cards (especially if you picked up a larger capacity card like the MC-251 or the MC-1019)... which is why game consoles of the era (PS1 & PS2 most notably) had 2 card slots.

It's for you to have sex since you're such a loser that no girls will have sex with you.

So you can have two memory cards.

just extra storage