I'm going to start off by saying that if you don't exercise at least a little already; go and do so! At least 30 minutes a day is recommended for maintaining your weight and improving your health.
So assuming you already do exercise, why mix it up? There's a fabulous article at The Bone Architect blog about this very thing. Briefly, your muscles and bones get accustomed to what you're doing, so it's best to switch things up to keep challenging your muscles to stretch and grow. An easier way to think of it is that you have hundreds of muscles and bones in your body. If you just do a single type of exercise (only walking on the elliptical machine or only using the rower, for example), then you aren't actually working out your whole body. So certain parts will get stronger and more toned, but others will stay weak.
Plus, doing the same exercise routine all the time gets REALLY boring for me. I can't stand to do the same thing over and over, so by varying my activity I make it more likely that I'll actually do it!
So how can you mix up the way you exercise?
★Try a different type, such as yoga, Pilates, or dance
★Include different exercise modes such as cardio and strength
★Try "pre-exhaust training"
★And when in doubt, walk!
So the next time you go to work out, mix it up and change your routine!