I hope you would have found answer for this, still, let me answer it. Reply, if you are not understood the topic.
When opening larger assemblies, the loading of components will take some time, since it has to load every information regarding each component.
When you open an assembly using graphics representation, the components graphics only will be loaded, not the geometry info of component (after opening the assembly, look the loaded objects in Folder browser ---> In session).
When you open in geometry representation, more items and geometry info will be loaded, so that you can measure a feature or part.
By these representation, you can reduce the opening time of an assembly, by preventing, unwanted loading of informations.