The presenting face of an OS is the management of the files (which include executable files, or applications, or programs if you prefer). Where they are within the volume & folder hierarchy, when they were created or modified, what size are they, and oh would you like to open/launch it?
Behind the scenes, the OS is comparing you (via your login account) to a table of actions and elements in order to determine whether you should be allowed to do certain things or which of a set of variable behaviors and appearances is appropriate to render or enable in this or that situation.
I could do all that in FileMaker Pro (albeit slowly and inefficiently, it’s way too high-level of a language for such a task). It’s all just databsase functionality, really.
Most current operating systems have, until somewhat recently, been rather clumsy and inefficient when it comes to managing files and resources for the user. A database would not be limited to displaying a “browse” view of files only within their actual folders — you could do a “Find” (or “Query” for you SQL types) and the results window would behave as if it were a folder or directory containing items, even though the real locations of those items might vary all over the place.
The modern OS gives you that kind of database functionality. From the “Results” screen you can rename files, move them to another location, toss 'em in the trash, apply a parameter (label, comment, association with a certain application, permissions restriction set, etc), and so on. And instead of some search engine having to crank up and go LOOK for files and resources that match the parameters you entered, the modern OS has it all indexed, so the moment you specify, the results are “JUST THERE”, bang. Easy as opening your Macintosh HD or C drive.