It's just a thing she enjoys/kink. A lot of people have some sort of kink/particular thing they like to watch/imagine.
There's not necessarily any rhyme or reason to it or way they can explain what they like about it.
If she were to try and make you do it in real life it'd be different, depending whether you like/could roll with the idea. But if it's just viewing/fantasy, it's harmless, although it can be unnerving to hear about other's kinks.
I suppose she might've told you in the hope that you'd be up for it though.