In addition to backing up saved games and Wii Channels/Virtual Console/Wiiware games (btw, DO NOT run Channels from the SD card; they have to be copied back to the Wii each time, which unnecessarily wears out the memory), and running homebrew applications, the SD card is also used by some games for supplemental storage. For example, Super Smash Brothers allows you to save levels you design to the SD card.