San Andreas Fitness
Throughout the game you need to keep Carl in shape. Going to a gym you can increase your stamina, muscle and you will be able to run faster. Not only will CJ get stronger and look better, but you will gain respect from your fellow gang members and the pedestrians on the street.
You could be lazy and not take CJ to the gym, but as you play the game you will realise that it is a necessity to go exercising in the gym. Exercises include using the Treadmill, the exercise bike, and building your stamina with the 2 types of weights. You can also learn some new moves here from time to time. There is a gym in San Fierro for instance that teaches you martial arts.