It helped you understand why dropping app bundles in the trash does not clean out per-user property lists.

App bundles are never installed anywhere as they are self-contained products, hence they cannot be uninstalled. For applications that actually come in the form of package installers which may place files in "annoying" locations like /usr/local/, you also get the functionality of uninstalling the software, because these packages contain manifests describing what files go in what locations.