We reported on the Switch's 4.00 firmware update last night, but in typical Nintendo fashion, their firmware notes did not fully explain everything they added. Also in typical Nintendo fans, some of the things they added do not fully do everything you would want.
The Nintendo Switch subreddit, a community once known for its rabid fervor for NX news to the point where posters were encouraged to post their dreams about the system, discovered last night that USB headsets being plugged into the dock were recognized by the Switch. Those with USB-A to USB-C adapters were even able to plug directly into the Switch for the same result.
USB speakers also work similarly.
So how does this benefit you? Well, you have to be in a pretty specific situation. You might have headphones you really like but are USB and don't want to get an adapter for the AUX port. You can, as we mentioned, use USB speakers that might be better than your TV's or for when you are doing local multiplayer in tabletop mode. You can repurpose old headsets that you don't have any need for but want to use for game audio on the Switch.
What it will not do is use let you voice chat. Despite having the capability with the type of hardware it uses, the headphone jack and now USB headset compatibility, the Switch OS will not allow voice chat through the system online. That still requires the Nintendo phone app, which is at best cumbersome, and only recently updated to allow for your phone falling asleep.
It is interesting that Nintendo added this feature, though it does not seem like it will benefit that many people. Still, maybe it means they will eventually reconsider their strategy with the phone app and allow people to talk directly through the console, or are at least open to the idea.