As KAD mentioned, rugs without rugpads can be slippery. Especially if you have them on wooden or tile floors (and not on wall-to-wall carpet, which I personally find abominable, but it's like a wall-to-wall rug pad, I imagine), and children, older people or anyone careless living in your house or visiting, they can easily slip on the rug (the rug moving with them when they move fast), and fall, and sustain serious injuries.
On a lighter note, my cat used to "rug surf" when she was younger: I had this 3x6 fine weave Balouch in the hallway, and she would run towards it, then jump on it and slide with it a foot (30cm) or so. She'd do that again and again. With rug pad impossible.