You should always take the time for a quick warm-up, even when you don't think you have the time. The reason? If you start your exercise without first warming up your major muscle groups, you're putting yourself at risk for injury and will not perform as well as you would if you'd taken the five minutes to do those lunges, jumps or high-walks.
When you warm up, you start to raise your core body temperature and heart rate, which gets blood flowing to your muscles and helps loosen up your joints. Many people make the mistake of confusing warming up with stretching: You should not stretch before you exercise for the simple reason that your muscles aren't very pliable yet. Not only will you get a better stretch at the end of your cool down, but you're also less at risk for injury from straining a muscle that's not warmed up. Most fitness classes begin with a group warm-up, but when you're on your own or being called on the court by your friends, it's best to take the extra five minutes to get your blood flowing.
You want to cool down for the same reason as you warm up: Your body needs a transition zone between being sedentary and very active. When you exercise, blood courses to your muscles. If you cease your activity abruptly, you don't give your body a chance to shift into a more appropriate heart rate, so blood can pool in your muscles and cause a fair amount of pain.
Stretching can make you more flexible, increase your range of motion, prevent future injuries and relax you. You should always make sure your muscles are warm before you stretch. Be sure to stretch all major muscle groups. Pay special attention to those you've just worked.