Not to belittle it, but can you explain how this is different from selecting "Games" in the search filters?
The main difference is that it will still display DLC for games you
do own, while filtering out the DLC for games you
don't. In the instance of Europa Universalis, I own III but not IV. If I wanted to search for DLC for III but not IV, there currently isn't a way to do this in Steam.
Additionally, one could leave the search field blank and select DLC from the filter list to pull a list of only the DLC on Steam for games they already own.