Flex Layout is just a Layout only with 3 children. Middle child has flex grow is set to 1. Other children have flex grow set to 0.
It is observed that this type of layout is predominantly used in mobile apps. Instead of developer creating it everytime from the generic Layout widget, a simple drag and drop option will be helpful to developer.
Layout and Flex Layout are available only for React Native apps. Cordova and Web apps will be supported in future.