After Nintendo shocked fans with the announcement of the Switch's upcoming online service, we learned that an upcoming app on the smartphone would control most of the online features. In an interview with GameSpot, Nintendo president Reggie Fils-Aime discussed the the app in detail. See Reggie's direct response:
The smart phone app that we’re creating, that will be part of our online service, we believe is going to be a very compelling part of the overall proposition because that’s how you’ll voice chat, that’s how you’ll do your matchmaking, and create your lobby. We also think it’s a very elegant solution because if you’ve taken your switch on the go, you’ve put yourself in a hotspot, you’re looking at get a quick match of Mario Kart in, to whip out some sort of bulky, gamer headset is a bit of a challenge.
So we think we’ve got an elegant solution. That’s a very specific answer to your question. - Nintendo President