Cycling involves long periods of time out in the elements. Protection from harsh sunlight and the immediate comfort provided by sunglasses will be the main reason people choose to wear them whilst riding. However, cyclists often underestimate the value that cycling sunglasses offer in terms of safety whilst on the bike. Sunglasses protect the rider's eyes from foreign objects, which is is fairly high risk over the course of a few hours our on the road or trail. Particularly whilst travelling at high speed, the last thing you need is a bug or piece of dirt entering your eye when you have your own safety on the bike to concentrate on.
In terms of sun protection, the benefits are more universally understood. Sun exposure over prolonged periods is not just uncomfortable, but also damaging to the eyes. UV protection is is the key benefit here, as it will both help prevent eye-fatigue in the short term, and serious damage in the longer term. It should also be noted that UV radiation increases around 15% for every 900m gained in altitude, so cycling in mountains or areas high above sea level require even more precaution.
99 Bikes has a large range of cheap cycling sunglasses from leading cycling brands such as BBB and JetBlack.
Cyclists have particular requirements which make wearing cycling specific sunglasses the best option for protecting your eyes. Cyclists need to have good peripheral vision, which means a wrap around style is best as it provides minimal obstruction. Wraparounds will also keep drying wind off the eyes, as well as allowing sufficient air circulation to prevent fogging as you start to sweat.
More advanced sunglasses for cyclists will also feature interchangeable lenses, which give the rider options for different levels of light. For example, in low light conditions the rider might use lenses that enhance the light and improve visibility, while changing to heavily tinted lenses on a sunny day.