I've actually had some conversations with people about this kind of thing, and I've heard some interesting theories that I agree with. For this kind of thing, someone trying to get nudes of someone they know (even if only on the internet or in passing) rather than just going and finding porn, I'd say it's less about control and more about familiarity. Pictures from some professional model or porn star or whatever are so very impersonal, such that they might as well be fictional people as far as you're concerned, because you'll almost certainly never have any interaction with them. There's an appeal to familiarity, to being able to entertain the idea that your dirty little fantasies actually have a shot of coming true, and that can make it more exciting and enticing. It's similar to how being attracted to some celebrity is one thing, almost an academic interest in their looks or personality without much real emotion or passion in it, but having someone in real life that you can actually talk to and interact with that you are attracted to gets the blood pumping and adrenaline flowing.

Related to the familiarity thing, there's also probably some thoughts going on there that if someone is willing to send you racy pictures of them then they're probably attracted to you as well. After all, why would someone give you these intimate pictures if they don't want to get intimate with you? This is kind of naive, and there are plenty of people out there who just don't give a fuck and will send out nudes to curry favor and such, but it's very likely a thought that goes through the mind of someone asking for another person to give them pics of their body.

Another factor is exclusivity. Any chump can hit up Google and find images and videos of whatever porn star they like, but the same likely can't be said of that girl that you're chatting up on Facebook. People like being the only person to have something, or to be one of a select few, and that most certainly applies to this context.

All that said, yeah, there are also plenty of people who like having some kind of control over others. Some people get a thrill out of using others regardless of what it's for, because while it may be kind of ridiculous or take more effort, they'll likely see it as being worth it for that extra pleasure they get out of having control over someone else. I'm just saying that there's more to the example you've given than pure controlling desires.