A slip can be used in many ways

Great for wearing under transparent dresses or skirts and they make the fabric of your clothes fall nicely.
If you wear tights, a slip can stop your dress from riding up or sticking to your legs.

A silk slip can help you stay warmer in winter and cooler in summer. You can wear a slip as a flirty nightie.
And, last but not least, a slip makes you feel more feminine and sensual.