My primary reason for getting one was to be able to play third-party translated Japanese games on my real TurboDuo (or SuperGrafx). Also, independently released games like Tongueman's Logic which isn't available in ISO format for burning to a CD. Otherwise you'd be stuck playing these things in an emulator on your computer.
Blue Blink, Bubblegum Crash, Wonderboy in Monster Land, etc....
Good stuff!
You can also use them to play homebrew tech demos (such as Chris Covell's stuff and the stuff you find in Zeograd's Lair) on the real hardware. They interface with your computer via USB and transferring ROMs is quick and painless.