Why companies rate UPS in VA's and PSU's in Watts

Almost everyone who has went to buy power equipment such as a power supply and UPS has assuredly noticed that while power supply units are rated in Watts, UPS systems are rated in VA or in both VA and Watts. This usually leads to confusion as most people not only lack the technical knowledge to understand the difference between VA and Watt units but few might even realize that there is a difference.