Caffeine is a stimulant for the nervous system, heart, and muscles. Research has found that having caffeine before your workout, can increase the number of calories burned during your workout. It also decreases muscle pain while you exercise, and it increases the amount of time you can work out before your muscles get tired. A study done at the University of Nebraska found that participants who had caffeine before a workout were able to bench press up to 5 pounds more than on a regular basis.
You should be mindful though, of how much caffeine you have as it IS a drug. Too much caffeine can have side effects including anxiety, digestion issues, and high blood pressure so as with everything, moderate your intake.