“In our latest system software update, it enables third-party peripherals to work on Nintendo Switch. As a result, the peripheral for Wii U that allows it to attach GameCube controllers also works [...] I think it’s fair to say [...] that that peripheral working was as much a surprise to us as it was to consumers.”
There's no direct use for the controllers, but if you fancy playing games without the ZL button, it's as easy as plugging in the adapter, then the controller, then telling the Switch to pair with it. Joe Skrebels is IGN's UK News Editor, and he's do a no-shield run of Zelda with a GC controller if he wasn't such a coward. Follow him on Twitter.