When Windows doesn't have an association for a particular file, it gives you an option to use the Web service to find the correct program.
Where does that list come from? How does Microsoft know that your program can open the file? At least publicly, this goes by the very descriptive name "Windows File Association System on-boarding process." Essentially, you submit your file information to Microsoft, and they will add that information to their database. So instead of seeing
you can instead see
This is a great lead generation tool. You have someone who has a document created in your software, and they want to open it. Your marketing friends will love you.