Nintendo's decision to eschew a standalone Wii hard drive for the support of SDHC was a good one, as users can buy tiny and cheap 32GB SDHC cards to expand storage. But just what you could store was in question.
For instance, you could certainly store small, downloadable Virtual Console games and stream them from from the card. But what about full-game expansions, gigs of data? Traditionally, these couldn't just load from SD as you'd expect.
According to IGN, Nintendo has opened SDHC storage to developers for DLC that can boot naturally with the game, straight from SDHC with no hard drive middle ground.
The reason you'll want a new SD card is pretty simple; SD streaming will kick off with GH5. All the tricks used for both Guitar Hero and Rock Band in the past—leaving internal memory open—is gone. Instead, straight-up booting off the SD card is available, and that means easier downloads, larger expansions, and quicker loads when bringing in song data from the SD card into GH5.