There are cartridges which allow you to load games onto a micro SD card. They can be used to load homebrew games or roms obtained from less than legal means.
I had one for the original DS and obtained ROMs for the cartridges that I owned. It was convenient to have them all in one place when traveling without having to bring a bunch of cartridges with me.
Even better, if you load custom firmware[1] on it, you don't need the special cartridge at all, and you can dump your game cartridges right to the system's SD card (this only works for GameBoy Advance and 3DS games, but not original DS games). If you buy digital games from the eShop, this also gives you an effective way to back them up, including save files.
That's true, but it requires TwilightMenu++[1] and is not as straightforward as the usual method for 3DS and GBA games. Also, I don't think DS games will show up on the home screen like 3DS and GBA games do.
I had one for the original DS and obtained ROMs for the cartridges that I owned. It was convenient to have them all in one place when traveling without having to bring a bunch of cartridges with me.