Several standard Visualforce components allow for facets, such as "header" and "footer", to be inserted into their component bodies at specified locations --- much like Visualforce templates allow for apex:define sections to be included with the apex:insert tag.
Why can't our custom Visualforce components have custom facets?
This would make Components dramatically more useful, combining the usefulness of the apex:insert tag available with VF templates and merging it with the power of Components. It's extremely limiting to be only able to insert ONE apex:componentBody tag into the body of your Component. We could, instead, have the following: