Preworkout supplements are used prior to training mainly to improve exercise intensity, focus and endurance. However, some pre training supplements are designed instead to increase vascular pumps, reduce muscle tissue damage during training, preserve amino acid levels in the body or to increase the time a muscle can be stressed before fatigue induced failure occurs.
When choosing a pre-workout its important to keep in mind which of these things is most important to you as no one product delivers all of them. In fact, some of these goals are mutually exclusive, so by boosting one, you cause another to be slightly hampered. Definitly a case of no one pre-workout being best for everyone!