Hello Corz!
It could be a lot of things, she's really good in bed, she gives good care or she can make the men she has feel special and appreciated. However, this is just a theory. When you said that she is not beautiful and doesn't have a good figure, I think men doesn't always look for the physical appearance. Sometimes, they just want someone to care for them and give them sex whenever they want. For married men, I think when they look for a girl, they look for what is missing in their marriage (sex, intimacy, cuddling, etc) and your niece could be a good giver of whatever they were looking for, so it's not always about the physical appearance.
In other countries, there are websites you can go to, to look for a sugar daddy, yes it is a business. Both parties know that one is just after the money and the other one for sex or whatever they want. What would happen is that they they would sign up in a website, put up photos and upload what they want in a man. And these so called "sugar daddies" will message them, probably meet up for dinner and negotiate their terms.
It sounds weird but that's how the world is now. Even relationships has a business side.