Games and good marketing are both correct awnsers. Also, it helps that Switch has the best value proposition of any Nintendo product in more than a decade.
As a customer you'll get one product that is both an high-end handheld and a competent home console to play the latest Nintendo games. You have basic couch multiplayer right in the box and a great catalougue of games to play with friends. You have access to a great selection of indie games playable on the go, as a plus you'll even get to play some PS/XB multiplatform titles with some minor drawbacks.