Coverstitch machines are primarily used to create professional-looking hems to garments. It has the dual function of covering raw edges of a fabric and also retaining fabric stretchability. You will commonly find coverstitch on sleeves hem, pants hem and necklines.
Yes, you can, and truth be told, you need both. There are machines that do both, and they're usually called a combo machine, meaning a combination of a serger and a coverstitch. However, many sewers prefer to have two separate machines.