There is no shortage of messaging apps jam-packed with features to help you stay in touch with friends. Alphabet Inc.’s Google has a new app, called Duo, that is going in the other direction and focusing on just one thing: video chat. Is it enough to get you to switch?

That depends on whether you chat often enough through video, and whether you prize simplicity. If the answers to both are yes, Duo is worth a look.

Duo,...