Every time an image is pinned, Pinterest generates a set of resized copies and stores them alongside the original. You can see this in the image address itself: the URL contains a segment such as 236x, 474x, 736x or originals, and swapping that segment changes which rendition you receive. The grid view loads the small ones so a board scrolls quickly, and the closeup view loads a mid-size copy that looks sharp on screen but is nowhere near the uploaded file.
When you right-click and save, the browser writes out whatever it already has in memory — which is the copy the page decided to load. Almost always that is the 736-pixel version. On a phone screen the difference is invisible. Put the same file into a print layout, a presentation, or a design at any real size, and it falls apart, because you are enlarging an image that was already reduced.
Fetching the originals rendition avoids all of that. It is not an enhancement or an upscale — no detail is invented, and nothing is processed. It is simply a request for the file that was already sitting there, at whatever size the pinner uploaded it.
That last point is worth being honest about. Pinterest never upscales, so the original is only as large as the source. Plenty of pins were themselves saved from somewhere else at low resolution before being uploaded, and for those the original is small too. We show the real pixel dimensions before you download so you can judge whether a pin is worth saving, rather than discovering it afterwards.