It is indeed a very large download - none of which is used unless there are errors found in the installation. If errors are found, then it attempts to source the necessary files from the store it may create during the 'install' - and then deletes the store when it exits.
It doesn't actually install anything, really - it merely uses the installer framework to test and repair failures in other patches/hotfixes which MS has determined are common problems, from the data returned from the 'Customer Experience' program which sends anonymous data back to MS to allow them to monitor how Windows is performing generally, and Updates in particular.