Numerous factors need to be considered when deciding whether or not it is necessary to purchase an all-weather cover for your garden furniture. The first and most important thing to appreciate is that some types of outdoor furniture will require a protective cover more than others.
For example, garden furniture constructed from man-made materials such as plastic or metal generally requires less protection from the elements than natural products like wood or stone. Good quality cast aluminum (metal) and woven (plastic) furniture can be left outside throughout the year without a cover. The appearance of the metal and plastic outdoor tables and chairs will not significantly alter when left outside uncovered.
However, when left exposed outside teak (wood) and granite (stone) furniture can look different over a period of time. Teak usually becomes more silvery grey and granite often becomes less vibrant in color. To prevent this most suppliers of outdoor furniture will recommend that the furniture is either taken indoors, treated, or covered when not in use.
Nonetheless, many people actually prefer the look of weathered teak and therefore leave it outside untreated and uncovered. Providing the teak is good quality then this decision should not significantly affect the life or durability of the furniture.
Another significant factor to consider is that each garden is subjected to unique weather conditions. For example, south-facing gardens are more exposed to high levels of UV which can make certain types of furniture, particularly wood, change its appearance quicker. Consequently, a cover should be used so that the weathering process can be delayed if the original look is preferred.
Covers are also often used to prevent the garden furniture from getting dirty. Obviously, if the garden is full of birds, trees, and other vegetation then the outdoor dining set will need cleaning more regularly unless it is covered.
In summary, it is down to personal choice whether a cover is used or not. A protective cover is certainly not a necessity in order to extend the life of garden furniture. Many people prefer the weathered look, have storage space, or feel that a cover is simply not required. Whatever the decision, as long as the metal, wooden, plastic, or stone garden furniture is good quality then it should be durable and last for many years.