1. Burn More Calories. When you do the same exercise for a long period of time, your body gets accustomed to it. You learn the movement and become more efficient at it. If you were a car, increased efficiency is a good thing; you burn less gas doing the same work. Since you're human, increased efficiency means that over time, you'll start to burn less calories doing the same amount of exercise. You can avoid this by participating in different forms of activity.
2. For Fun. Doing the same exercises over and over again can get boring. If you stick to the same routine for too long, you'll get sick of it and increase the chances that you'll stop working out all together. Exercise is like a popular song. At first you can't have enough of it. After it gets played on every single radio station at the same time, you start getting sick of it. If you do the same exercise again and again, you'll get bored. Switch your routine before you get sick of it so that one day, you can go back to it and still enjoy it.
3. Work Different Muscles. If you do the same exercises, you'll always be working your muscles in the same way from the same angles. New exercises that offer slightly different movements will help your muscle get strong in more than one range of motion.
4. Exercise Plateaus. If you're stuck on the same weight or number of repetitions for a long period of time, you're probably experiencing a plateau. A good way to get over this is to change your routine around and try some different exercises. This will force your muscles to adapt to new ways of moving and will allow you to break through your plateaus.
5. Meet New People. If you're into group exercise classes, take a few that are different than the ones you normally participate in. If you always spin, try yoga or pilates instead. You'll get a different type of workout and meet some new people.