Assuming that each of you have been that woman at some point or another, let me make the case for slips.
First, you can wear any underwear you want! Well, almost any. Unless your dress is basically sheer, the additional layer will keep printed panties from showing through. For busty ladies, the slip will also provide additional support up top.
Second, it’s helpful in any season. In the summer, a slip will keep your dress or skirt from sticking to your sweaty legs and butt. No more peeling your dress off of yourself when you stand up — the slip ensures it falls nicely. And, in the winter, a slip will keep your skirt from sticking to your tights. Also helpful.
Third, while you might think of slips as a girdle-like retro item, they really have the opposite effect of most vintage lingerie: They keep you comfortable because they allow your clothes—and therefore you—to move more naturally. A slip keeps cottons (and other fabrics without great movement to them) from bunching up weirdly. In general, your dresses and skirts lay smoother and move more freely as you walk.
Fourth, they are sexy as hell.
You can find a few basic slips in the lingerie section of most department stores, but most new slips are pretty plain and prim. They’ll get the job done. However, to really have fun with it, do a quick Etsy search for vintage slips, and you’ll find a treasure trove of great lacy details and fun colors. Slips are labeled by bust size (so you can be reasonably sure you’re getting a good fit) and are pretty cheap, to boot (so it’s not a huge commitment). Give one a try. I’m willing to bet the slip could become a wardrobe staple for you, too.