This entity contains necessary information to be able to construct a document. It has the base or master view file, as well as some optional blocks as content.
Content in web pages are build using blocks. The Block is the model in an MVC-like structure where View is a twig file that the Controller BlockService will reference.
This class works like a controller for the block and simultaneously expose methods to provide FormTypes for content management purposes.
The BlockContainerInterface is used to identify classes that can hold Blocks as content. Usually these classes are directly addressable and viewable like Pages.