All the posts contributed thus far are good examples of various applications for the SD card slot. Mine differs slightly, in that I put a spare 16GB SD card I already had "lying around" into the slot "just in case."
I always shoot to a fast 16GB CF card, and I usually carry a wallet of extra CF cards with me. But I like the "extra insurance" of being able to switch over to the SD card if the CF card fills up or fails (never had a CF card fail in 15 years), or -- given the need and enough time -- to copy the contents of a full CF card over to the SD card, then erase / reformat the CF card and resume shooting.
I don't think I would spend the money to buy an SD card for my 5D3 if I didn't already have one, however, just to fill the slot.