The defining aspect of the flex layout is the ability to alter its items’ width and/or height to best fill the available space on any display device. A flex container expands items to fill available free space, or shrinks them to prevent overflow.
The flexbox layout algorithm is direction-agnostic as opposed to the block layout, which is vertically-biased, or the inline layout, which is horizontally-biased. While the block layout works well for pages, it lacks sufficient definition to support application components that have to change orientation, resize, stretch, or shrink as the user agent changes, flips from vertical to horizontal, and so forth. Flexbox layout is most appropriate for the components of an application, and small-scale layouts, while the (emerging) Grid layout is intended for larger scale layouts. Both are part of a wider effort of the CSS Working Group to provide for greater interoperability of web applications with different user agents, different writing modes, and other demands on flexibility.