I absolutely understand the annoyance-factor of "Pinterest-itis" (though it seems that either they've dialled their "you must absolutely register now in order to see anything"-behaviour down a little bit again, or maybe it's just that half-forgotten userscript I installed at some point – either way it's currently not
that infuriating for me), but given that e.g. direct image indexing seems almost non-existant for Instagram, being able to stumble across some images from there via the detour of Pinterest is better than nothing I suppose. (It's only infuriating if somebody saved an image to their computer/phone/… and then manually re-uploaded it to Pinterest again, so that the source URL of the image is then missing…)
Likewise if the original image has already disappeared from the net – finding a copy via Pinterest is still better than finding nothing. (Interestingly Bing's search can serve a similar purpose – compared with Google, it feels like Bing is much more reluctant to remove 404'd images from its index again. While those broken results are annoying, too – and unlike Pinterest neither Google nor Bing store (at least publicly) full-resolution copies of those images – at least you can try plugging the preview image into reverse image search and with some luck maybe find an alternative copy of that image that has survived elsewhere.)
And finally, the keywords added by Pinterest are possibly also helpful in making images more discoverable. (This might be no longer that relevant now that Google is also running image recognition on the images it indexes and no longer has to solely rely on the hope that any text near that image or the rare "alt"/"title"-text is actually relevant to the image contents, but it might still help a little.)