Well, it is for sorting purposes. (They've got massive machines running Linux doing OCR which replaced manual sorting, and that requires... taking pictures of the mail.)
Right, but then the USPS was claiming that they simply threw away all of the resulting data when they were done with it. That's a ridiculous claim in every way.
Whether all the pictures are also retained is a completely different story. 10 years ago, I'd have said, "No; too expensive." But storage costs have plummeted, so nowadays, maybe so.
So what? They don't have to OCR anything that has a properly printed label; they just can it for bar codes. Those pieces of mail, which are the bulk of what passes through the postal system, never has to be photographed at all because they already know where it's coming from, where it's going, what it weighs and whether the package weight was reported accurately. The scans of the remaining minority of mail could quite reasonably be saved ten years ago, especially if you were not picky about resolution. Today, it's trivial.
But the real "so what" is that they are OCRing the mail, so even if they were throwing away all of those scans, they would still reasonably be storing the metadata. Why would you ever throw that away, unless forced? It's small, and it's valuable. But moreover, one of the Snowden revelations was that they are in fact storing all of that OCR data, it all gets handed straight to the feds. Before Snowden, it was generally believed (heh heh) that this data was simply flushed, and only the fringe believed that it was handed to the feds as a matter of course. Now we know that to be the case.