Pre-workout caffeine consumption has been shown to reduce perceived effort and fatigue during exercise.
This helps explain why caffeine consumption before training generally results in improved workout performance.
If you decide to start taking some per-workout caffeine in the form of coffee to pre-work out supplements consider the following. Proper caffeine dosing will depend on the individual (mostly due to individual caffeine tolerance), but around 3-6mg/kg (1.4-2.7mg/lb) of body weight prior to training has been suggested to be a good range for most people seeking a performance improvement.
Considering you can build a tolerance to caffeine, it's a good idea to be strategic about your caffeine intake.
Ideally, limit high-caffeine consumption to days when you have low energy levels but need to train or before high-volume training days.