The purpose is probably to make prey more complaint. But a vampire with the ability could tell someone to make them a smoothie or demand that the person kill themselves. I don't like the ability.
[Everyone within her home world has assumed that she'll use her ability to compel them to do what she wants. They'd even questioned her on why she didn't. But it's obvious, isn't it? It's frustrating to feel helpless. No one wants their free will taken from them.]