Why so many files in /usr/share/app-install/desktop?
The .desktop files under /usr/share/app-install/desktop are installed by the app-install-data package. This package provides metadata about applications that is used by the Ubuntu Software Center and (in your case) the Lubuntu Software Center. The package also contains a copy of each application's icon that is referenced from the
.desktop files.
Basically, these .desktop files are very similar to the .desktop files that are used by your desktop environment's menu for launching applications. However, as you noticed, they have additional metadata added for things like popularity counts and search keywords.
And, as is probably obvious, the app-install-data package installs a full set of .desktop files and icons on all Ubuntu systems regardless of which flavor is installed, which applications are installed by default, or which applications have been installed or removed on a particular system.
The purpose of all of this is to give users a polished Software Center experience that provides information about all applications that can be installed in Ubuntu without having to query and cache all of it from a web service. The app-install-data package is built for each Ubuntu release from all known applications that are in the Ubuntu repositories. And that allows the user to open the Software Center, search for applications by name or keyword, and be able to see the same name, description, and icon that will show up in their desktop menu or launcher when they install the application.
These metadata files and icons can be removed if you want by simply uninstalling the package that contains them:
sudo apt-get remove app-install-data
Note that removing app-install-data will in turn force removal of lubuntu-software-center, lubuntu-desktop, software-center, and ubuntu-desktop. However, removal of these packages will not affect Synaptic, aptitude, apt-get, or any other front-ends for the apt package manager. Only the Ubuntu Software Center application requires these files.