If you put furniture outside in an area where sun throughout the day is an issue, you need your cushion coverings to be sun resistant. Luckily, polyester is available in varieties that withstand fading from the sun very well. Not all polyester is created equal, however. Look for varieties that are listed as being sun resistant or UV treated to be sure that the fabric won't fade.
Polyester fabric is very easy to clean. This makes it a good option for outdoor cushions that will probably be covered with dust, dirt and other outdoor grime regularly. In addition, most outdoor furniture is used as part of an eating area, making more stains and messes a real possibility. Polyester cushion covers can be thrown in the laundry and are naturally difficult to stain permanently.
Moisture and water is an ongoing concern for outdoor furniture. Even if your furniture stays out of the rain, condensation is likely going to build up on the furniture regularly. Polyester is not permeated easily by water. Secondly, polyester resists the types of molds and mildews that will take root in fabrics made of natural fibers. This means the furniture will have a longer life if it is made out of polyester.
Polyester is a good fabric to use in any high-traffic and high-use area. First, repeated washings don't make polyester weaker the way other fabrics become when washed too often. Polyester also doesn't wear the way other fibers do. This means that it can be used heavily as seating without stretching out of form and becoming weak over time.