Foldable phones have been around for almost half a decade, but they still account for only a small portion of the mobile phone market. There are a few good reasons why: Such phones are expensive, less durable than standard mobile devices and have a visible crease running across their displays. But most importantly, foldable phones still lack a so-called "killer app:" An app or feature that defines their purpose and is compelling enough to convince people to buy them. With new foldables like the Samsung Galaxy Z Flip 5, Motorola Razr Plus and Google Pixel Fold, tech giants are taking...