Science has told us it's important to warm up before any sport and speaking for myself I almost always find I ride stronger after warming up gradually. Cycling and as well as running use very specific muscle groups in a very intense manner so the chance of injuring those specific muscles becomes quite high, whereas many other forms of exercise are less intensive and "share the load" across the whole body lessening the chance for injury. If you allow your heart rate and core temperature too gradually increase, your body and mind will be ready for the challenges of your ride.
Physiological importance of warming up:
Increased the muscle temperature. A warmed muscle both contracts more forcefully and relaxes more quickly. Speed and strength can be optimized.
Blood vessels become dilated. This lowers the resistance to blood flow and lowers stress on the heart.
Facilitates oxygen utilization by warmed muscles because hemoglobin releases oxygen more readily at higher muscle temperatures
Lactate resynthesizing mechanisms begin to engage during warm up allowing lactate threshold to increase normally.
Improved Range of Motion. The range of motion around a joint is increased.
Mental prep. The warm-up is also a good time to mentally prepare for performance by clearing your mind, increasing focus and concentration. This can be just as important as the physiological effects.
20 to 30 minutes is considered an ideal amount of time to spend warming up prior to the main portion of your training or racing. Spin easy for 5-10 minutes, and then slowly build up to threshold. Spin easy, and then do two or three 1 minute efforts just above threshold efforts. Spin easy till it's time to start. Some recommendations on warm-ups get far too complex.