As you wear your contact lenses proteins and oils from the eye deposit on the surface of the lens. While cleaning the lenses every day will help remove some of the deposits, over time they build up on the surface. As these materials break down they effect the surface of the lens reducing comfort and quality of vision. They can also cause the eye to develop an allergic or immune response resulting in sore, red eyes.
Replacing the lenses as recommended by your optometrist will ensure that you always have comfortable vision and minimize the chances of developing any problems.