My SD cards contains downloaded games, photos, and music. (The music is very rarely used.)
There is one downside to using an SD card to store games: when you start a game from the SD card, it takes between 5 and 15 seconds to copy over to Wii system memory. The benefit is that you can download and hold many more games on an SD card than you could by just saving them directly on your Wii.
Since I have about 60 downloaded games, the SD card is necessary for me. The short wait before starting a game doesn't really bother me. However, if you only plan to download a few games then it makes sense to save them directly on your Wii - if you run out of memory later, you can buy an SD card and move the game from your Wii to the card without losing anything.