When a rug is on a laminate floor, using the proper rug pad will prevent damage to the floor. Depending on the type of area rug on the floor, the rug could either have a rough textured back or it could contain a solid latex backing. In either case, damage can occur to the floor.
A rug with a rough backing can scratch the floor when pressure is applied, such as pressure from foot steps or furniture. This pressure causes the back of the rug to rub against the floor and this can eventually scratch the floor or remove the finish.
There are rugs that contain a sprayed on latex backing. Here, the latex can literally stick to the floor causing very expensive damage. A solid rug pad is useful to act as a barrier and prevent any latex from sticking to the floor.