Load as Group gives you the ability to load a Revit project as a group into the current project.
The project that you are loading (as a group) may contain various project parameters. Revit determines whether a parameter will come along for the ride based on if that parameter is actually in use. Two examples:
You have a typical unit .rvt project that contains walls, doors, furniture and casework objects. There are project parameters for those categories as well as parameters for floors, windows and generic models. When you load this project as a group the parameters for floors, windows and generic models are not loaded into the host project, because there are no matching objects of those categories (in the project you are loading as a group).
You have a typical floor layout .rvt that consists only of walls. There are 10 project parameters for walls. 3 of the wall parameters contain no data for any wall instance. You load this project as a group and note that only 7 parameters were loaded into the project. All it takes is 1 wall instance to contain some parameter value for it to be loaded.