Contact lenses can give us freedom from glasses, and great vision. The biggest problem with contacts is that they can interfere with the health of the eye, by reducing the amount of oxygen that is absorbed by the cornea (the clear window in front of the eye, where a contact lens is placed).
Eyes need to breathe, just like any other organ, similar to how skin needs to breathe to be healthy. When a contact lens is placed on the eye, it cuts down the amount of oxygen that is transmitted. As long as it is done gradually, and with healthy lenses, most people do not experience any adverse reactions. However, hypoxia, which is when the cornea is oxygen deprived, is one of the most common problems of contact lens wear.
Why wouldn't someone wear silicone lenses?
Rigid gas permeable (RGP) contact lenses are also oxygen permeable. Silicone lenses are the first soft lenses to compete with RGPs as far as oxygen transmissibility goes. Unfortunately, RGP's have more drawbacks than silicone lenses.
Oxygen permeable contact lenses are a very important feature for anyone who wants to wear contacts a majority of the time. They greatly increase health and comfort, and make lens wear easier and more convenient.