The Switch's companion app lets you view game stats and chat with your friends in Nintendo's new multiplayer game; support for other titles presumably will follow. However, the Switch Online experience is currently so bad that you're better off ignoring it in favor of a chat service that actually works.
Here's how Switch Online voice chat works right now. You fire up Splatoon 2, open up a very specific game mode and create a room. Then, you have to switch over to your smartphone and invite some friends. Hopefully, you have a lot of them, because the app only supports private matches.
Sounds annoying? It only gets worse.
To chat via Switch Online, the app needs to be fully open at all times. Want to keep chatting as you thumb through other apps? You can’t. Want to lock your phone and keep talking? Not in Nintendo's world. Unlike just about any smartphone communication app out there, Switch Online doesn't work in the background of whatever else you're doing.
This would be easier to swallow if there was some sort of in-game benefit to chatting via Switch Online. There isn't. In fact, it makes the Splatoon 2 experience worse, as you're limited in what game modes you can play and can't talk to pals that are playing other Switch games. The app doesn't let you view your Switch friends list or browse for digital games, which have been standard features of the official PlayStation and Xbox apps for years.
Until Nintendo fixes things up with Switch Online, you're much better off using fully functional chat apps like Skype or Discord to chat with your friends while you trade ink in Splatoon 2. As it stands, the only reason to have Switch Online on your phone is to access the handful of app-exclusive gear pieces available for Splatoon 2.
The Switch Online app is meant to serve as a preview of a more fully baked service, which is set to launch in 2018 for $20 a year and will be required for online play. Nintendo has a lot of work to do to convince us that Switch Online will eventually be worth paying for, so here's hoping things get smoothed out by the end of the year.