A well made awning will give you hours and even months of increased usage for your deck.
Whether it is really hot are even raining, you can have friends and family gather on your deck and enjoy the outdoors.
If you have large windows in front of your deck and especially if your deck faces south your air conditioner will be working overtime to compensate for all the solar heat. Protection from the sun over your deck will help mitigate this problem.
The beauty of a retractable shade system is that you don't have to go to the trouble of building a permanent roof structure. The retractable option gives you choice in both function and style depending on the weather.
An awning will attach to the side of the house whereas a canopy style system will typically be part of a structure like a pergola that is unattached to the home. Each provide protection from the sun in different types of scenarios.