I understand why it exists; I mean, it's the internet, it's bound to exist... hell, fanfic has been around since fiction itself has been present, so s it's likely to exist in actual publications, underground or otherwise.
It's also terribly fun to look up something gross, and spend literally hours looking at the hundreds and hundrerds of dickgirls, vomit, and ay other weird shit the internet can try to get you off with.
Actually discovered a few SFW artists whose work I enjoy through it actually.
Besides, 90% of Paheal's Featured Images have been... humorous at worst.
What makes it doubly ironic is that a lot of Rule 34 artists are actually quite talented, and/or work professionaly. (ie, they can use big words with "Printing Services" and "Commisions" non ironically, because fuck, those words pay for their food) and instead of making... art-art, they make Digimon-porn.