There isn't any use so far, doesn't mean there won't be any use later.
Also perhaps those panties are just a reference to D:OS1.
But anyhow, how should Dallis know, that you have her panties? Should she smell them?
She'd know if it was a specific design and/or material meant for her use, or if it was something she wore just recently. There's also the possibility that it was one of her favorite pairs, and that she's been looking for it specifically. You know how it is when it comes to people and their favorite things.
