On the list of things that men generally care about, new and good-looking underwear is right down the bottom in around the same position as quinoa. We just don't care - once you're down to your jocks, there's no going back anyway. Women, on the other hand, wouldn't be caught dead in granny pants when they're looking to get hot and heavy.
Sexy underwear has a way of making them feel hotter and more desirable, as much psychological as it is a matter of appearance. Putting some lingerie on is a statement of intent. So if you spot a few pairs of lacy new things in the drawer (and you're sure they're not to spice up your relationship), it's a good sign that she's expecting to get frisky in another bedroom.