The display of a non-interactive progress bar that reflects the amount of content that the user has read/scrolled through seems to be reflecting the trend that users want distinct visual indicators as a specific feedback to the information they are looking for, probably due to the amount of on screen interactions that they are bombarded with.
The last popular trend was to display the page or content loading progress towards the top of the page in a similar fashion to the youtube videos, and now I am noticing the display of progress indicators in long articles. I believe that these are commonly grouped together and referred to as "top progress bars".
Although some people might think this is rather redundant due to the presence of a scroll indicator on the right hand side of the page that provides the user with an idea of how far down the page they have scrolled, the content progress indicator is slightly more accurate as it ignores the banners and advertisements at the top and bottom of the page. As you can see in the image below, the user is nearly finished with the article, but is not near the bottom of the page.
Is this a useful or necessary aid that serves to improve the user experience, or is it really the result of too much clutter and 'stuff' that isn't related to the main content being presented to the user?
By the way, this feature is usually seems to be only present on desktop views and not on mobile.
UPDATE: This trend seems to be getting more popular despite previous answers questioning the usefulness of the UI element, and there are now more examples of how to implement this popping up everywhere: