Chaks recently wrote a post where he explained the difference between the two Microsoft Flow SharePoint actions Get items and Get files (properties only).
Behind the scenes they are both calling the following undocumented service which seems to wrap the regular OData call:
/_api/SP.APIHubConnector.GetListItems(listName='MyList',queryOptions=@q)?@q=''"
Good news is you can use both actions interchangeably. If you want to get the properties for a document library using the Get items action simply pick Custom value in the dropdown and enter the title or id for your document library and it works just fine (been using this already a while).
So, to me, having the Get files (properties only) action seems unnecessary, but for some users it might be easier to understand with two different actions targeted towards lists or libraries.