But if you want to play Splatoon on the go, the Switch needs to be Online at the same time. And if you want to talk to other people and not have them hear the game sound in the background, you need to have earphones with you.not being able to plug into the switch directly doesn't make much sense, but the phone hookup seems to me to be a way to create internet gameplay on the go. Like with a lot of things, this feels geared more towards japan.
This setup only gives you the option to play the game and use the voice-chat without headphones at all, which could also be achieved by including a build in mic.
So the reason for this setup is just to:
1. Saving money on a build in mic.
2. Saving battery-lifetime and RAM, since you don't need to have a program running in the background, which handles the voice-chat,
3. Saving bandwidth, since the voice-chat doesn't need to be send through the same network.
A really stupid case of min-maxing ...