Cloths can be used for many reasons; they help contain the mess that sometimes happens when we're creating, or a soft surface to place fragile items on, a way to protect a surface from dings and scratches, or help decorate a table top or shelf that maybe isn't the most visually appealing. If needed for folks that have to keep their altars tucked away for whatever reason, you can wrap up your altar supplies in an altar cloth and keep it safe when stored away. A cloth can be chosen based on colour correspondence for the seasons or Wheel of the Year, or even for specific intentions; an altar for protection, I'd use black; an altar for prosperity and abundance, I'd lean toward green or gold. There are cloths that are specifically designed for altars with pentacles, Celtic knots, astrological symbols and the like, printed, embroidered or sewn on them. It can also be a beautiful doily, a simple piece of fabric, or a scarf that's too pretty to wear but too magical to let just sit and collect dust.