Fap material, or simply just a strange interest or fascination. Although it's safer to assume fap material.
You've made it blatantly that you don't understand this fetish. And the thing is, there's no real way to explain it. I can easily explain why the fetish appeals to me, but you still won't get it, to put it bluntly. Your safest bet is to just accept that some people like this sort of thing, and move on. If you still want an attempt at an explanation, note me, and I'll try my best to make sense.
I am answering your question as best I can since I am giving you the privilege of the benefit of the doubt, and assuming this is a serious question. Don't abuse it.
"strange interest or fascination" huuuh... did not think that way? What if it is something like the pony brony thing? I would gladly hear your explanation, maybe it will make me more tolerant towards such interests I like your cool and mature answer , I expected more "gtfo" or "ffs"
With me at least, I'm actually more fascinated by the fetish than the type that just looks at these images to "get off" or "fap". And I wouldn't necessarily compare it the pony/brony "movement", since one is a fandom, and the other is a fetish. Not that I am discounting the weird side of pony fandom, I am fully aware of it's existence.
I think it's always important to give curious people a chance, since shunning behavior just offends me, and just gives another reason for people to hate fetish artists since they can be quite exclusive of others. That, and I enjoy answering questions.
Also, before I get any deeper into explaining this, would you be alright if I did so with notes? I understand if you would rather I did not (since I'm a internet weirdo, after all), but it will allow me to answer questions more honestly, since I won't be under the scrutiny of fellow "artists".