Watson10,
Protein supplements are just that, supplements. They are designed to provide users with pound-for-pound more and "better" (so they say) protein in your diet. They should not be likened to a magic pill as 2-LAP noted and the overall performance benefits for a cyclist are debatable (ricstern). They WILL, however, provide you with protein a cost effective, low calorie dose (powder) and normally require less time to prep and consume.
The key here is the word DIET. Normal diets for cyclists are not necessarily "normal". Normally one would eat 3 times a day about 500-800 calories a meal. Athletes/Cyclists may consume up to 6 meals a day and depending on the length of their workout consume up to 6,000 calories a day. An excellent balance of carbs, protein and fibers are the bare minimum for survival. Most of us also consume additional amino acids (such as those found in Cytomax etc) and electrolytes like the ones found in other sports drinks. It takes time and devotion to eat the right meals, in the right amounts at the right time. When you can not do that, you reach for supplements/replacement meals.
A protein drink with banana or protein bar can supplement, or replace, a chicken breast meal with rice. About .6 - .7 * (your weight in lbs.) is a healthy norm for good muscle support and recovery for an athlete. If you are not getting this from your "normal" diet, it would be helpful to:
1. Start eating/preparing more protein sources for your diet
or
2. Supplement with protein powders
While these diets have a tendency to be abused (Adkins et al.) and the fitness industry is making a mint off marketing the concept, the effects that protein have on a muscles are undeniable. They are the building blocks to muscles and protein does repair muscle tissue and fibers. Protein is not a good energy source as it does not burn well and requires a lot of water to break down.
Read up more about diet for cyclists and recreational athletes. Protein consumption is just one variable in the equation of healthy eating.
Ride with Carbs, recover with protein, carbs and fiber.
Good luck.