It is "also a service to provide cover art for file tagging". Just as MusicBrainz is also a source of information for tagging. See e.g., the discussion we had a little while ago:
But being an archive first means that priorities are slightly different from sites dedicated to music tagging cover art (such as fanart.tv).
Also, just for clarity: when you upload files to the CAA, you are not uploading anything to MusicBrainz. Any files you upload to the Cover Art Archive go directly to the Internet Archive's servers and never touch any infrastructure managed by MetaBrainz.
I think FanArt.tv is a great project, exactly because it gives a venue for putting out polished, standardised versions of cover art. But it is not a place to put original, non-touched up scans/images. So when cover art tagging requirements change (e.g., 500×500 used to be plenty, but these days even 1000×1000 are not enough in many cases), you will either have to dig up and rescan the original release… or find a source—and archive, if you will—that has this untouched original image, that can then be polished up again for the new standards. (Also, CAA is great for hosting scans of booklets etc., which are often the best direct source of information about MusicBrainz releases.)
What I'm trying to say is that I would rather have you upload squared front cover images to CAA along with all the original scans as well, rather than you not uploading anything at all. (In fact, maybe it is time for a discussion such as the one I linked but centered around CAA?)