to provide a generic entity browser/picker/selector. It can be used in any context where one needs to select few entities and do something with them.
Possible use cases: powerfull entity reference widget, embedding entities into wysiwyg,
8.x-2.x is a branch with support for the core media entity. You should only update to this version if you are using core version. If you are still using contrib version of Media entity keep using 8.x-1.x versions. If you are not using Media entity at all it should make any differenc