One of the purposes of rugs is to prevent floor from scratches. Now, how disappointing would it be to find out that the rug itself is scratching the floor? Dust, dirt, sand and other particles tend to accumulate on the rug, sift through it and land on the floor. If the rug isn’t properly attached to the floor, it will slightly move when people step on it. In the long run, a loose rug will have the same effect on your floor surface as a sheet of sandpaper.
You probably heard of pads that go under area rugs. Maybe you even used them. In this case, you surely know how important they are. They go between the rug and the floor, preventing the rug from moving. This protects your floor from scratches. Besides, a loose rug might cause falling accidents.
It’s worth mentioning that a big and heavy rug might not need an underlayment, especially if the room isn’t used often.
There are different types of carpet pad (also known as underlayment or cushion) and they are meant to be used with different kinds of rugs. At best, an improper rug-pad combination will result in poor traction. At worst, it might damage your pad and render it unusable in few months.
Also, let’s not forget that wall-to-wall pads are meant to be stapled or glued to the floor. Thus, they have no anti-slide properties.