The furniture will benefit from a wash down with soapy water once or twice a year to remove any algae or mildew especially if used on the north side of a building or under trees.
The furniture is pressure treated against rot and insect attack but will benefit from treatment against rain and sunshine to reduce small cracks known as checks. We suggest to protect your furniture painting them or applying a clear preservative. Apply a number of coats of paint or stain. Most paints/stains are now water based and will raise the end grain slightly, so a light sanding and possibly some exterior wood filler between coats will help.
We recommend from experience Colortrend Wood Coat (formally known as Solid Wood Stain) available in a large range of colours.