Boxing fitness is about rapidly getting fit. There are plenty of ways to work out a body and keep it fit - weights build up muscles and running, swimming and cycling are great for cardio. But the programs used by boxers to train themselves into peak physical fitness do all that and a lot more too, and you definitely gain more power with these intense workouts.
Boxing training uses sets and rounds of exercises to supply a full-body workout. These are set up so that you do a set of exercises that are full-on and elevate your breathing and strain your muscles, then you have a break, then another round of exercise. This mimics a real-life fight situation.
When you're straining muscles its important to vary the stresses to the different groups of muscles and let some rest while others are being used. This prevents any one group from getting tired or strained and being injured. The recovery time also helps the muscles develop faster. So sets of exercises are usually arranged to work-out different muscles at different times.
Interval Training
A range of sports use a method called interval training, which involves bursts of really high-intensity exercise alternated with lower-intensity or rests. It's been shown to get some really fast results. Runners frequently use interval training alternating fast and slow runs over the total distance. For strength and fitness, sprints or fast runs are generally more benificial.
Muscle Mass
Training and exercising in intense rounds often builds up muscle mass faster than weights, and a boxing fitness program provides high intensity phases that spur on the muscle development. The other advantage is that supporting muscles are also developed simultaneously resulting in less danger of injury - the smaller muscles can deal with the greater size of the main muscles.
Core Muscles
Boxing workouts often include developing the muscles at the front and back of the lower spine and pelvis. These muscles are called the core muscles and they maintain the correct bone and muscle alignment and prevent things from moving out of place throughout really active exercise. Exercises that help develop core strength are turning while pulling, toso twists, kneeing a punching bag and other exercises where you bring the knee up to the body.
Fitness & Self Defense
Self-confidence in an added ability to handle your environment is likely to improve as you get fitter and learn some self-defense. Self-defense might not be the primary goal of boxing training, but its a good added benefit to have.