If you have a very plain room, for example one which has been decorated entirely in the natural shades of cream and beige, cushions and area rugs are a great way of adding some interest to your room as well as making it more comfortable.
You can choose to add color with your cushions and rugs or you can add texture with them or (preferably) both.
One idea is to coordinate your cushions and rugs to bring in an accent color which adds impact to your color scheme yet keeps it coherent. For example, with the natural scheme you could add a chocolate brown rug and some rich satin pleated cushions in the same chocolate shade along with a few fluffy cream cushions and a couple of beige brocade cushions with beaded embroidery. Each element is adding color and/or texture while still being part of the natural color scheme.
This is much better in terms of interior design than cushions or rugs which fade away into the background. For example you could have cushions made up in the same fabric as your sofa and a rug which matches the color of your floor, for example, cream plain fabric cushions on a cream fabric sofa and a honey colored wool rug on top of oak flooring. You can see that this would add comfort to your furniture and flooring but it would have no impact at all.
If your color scheme is already more dramatic, perhaps you have a purple accent wall for example and a purple chair in contrast to your cream sofas then rugs and cushions can pick up on those colors you are using and tie your base color and accent color together. In this instance, you might look at purple rugs and cushions in the exact same shade as your wall and chair as well as cream cushions which add variety and texture and some cushions which contain both the purple and cream colors.