The purpose of romantasy is the same as the purpose of romance and/or fantasy: to model the relationships the readers desire in a world where those relationships are difficult to form. I don’t know what makes it more vulgar than any other romance genre. I do know that during my whole life, I have heard this sort of criticism lobbed at books that center women and love. I have not heard this criticism lobbed at books that center men and violence. It seems to me that fetishizing acts of war is a lot more vulgar than, say, summoning an interdimensional being and falling in love with it. And whether or not it is (very loosely) based on historical events, GRR Martin’s series is filled with vile, irredeemable people and stuffed full of the torture, mutilation, sexual assault, and denigration of women. But I guess that gets a pass, because it’s “true” fantasy.