Qigong (or Chi Gung) can be trained for many different reasons.  Where it be medical, spiritual, fitness, stress relief, health, or for martial development.  Though there is no substitute for sparring when learning to defend yourself there are many things that can increase your overall agility.  These may be cardio, strength training, and flexibility.  Picture qigong as another type of exercise.  If a professional boxer took out endurance exercises from their training, they probably wouldn't do to well. Qigong can make a very large difference within a martial artist of any background.

First of all Qigong training is exponential, what a practitioner gets out of a half hour in their first month will be nothing compared to what they get of the same time after 2 or 3 years.  This pushes many people away, but if you want to get the most out of your training or are in it for the long run Qigong can an invaluable training tool.

Qigong practice for martial arts training teaches a practitioner many things about their own body which they can use for self defense, while also physically developing the practitioner.  Through consistent practice the body learns to conserve strength and put less strain on itself, giving the practitioner more endurance and the ability to recover more quickly.   This increase in energy (or chi) can allow for other types of exercises (strength training, or cardio) to be done more regularly with less waiting period between exercises.  Qigong also teaches a practitioner to use and connect more of their muscle structure.  For example using structural alignment (the bodies correct posture) can increase a practitioners power and root (balance).  Qigong also helps a practitioner develop better control over differents muscles.  The exterior muscle groups are often lost at a very early age, such as the interior and exterior oblique's. By learning to utilize these muscles a practitioner can learn to develop a lot of power in Li or pulling techniques.   Chi gung also develops sinsitivity, increasing awerness of touch, this can allow a practioner to control their opponent according to where the oppenets wieght or force it directed.        

Besides what you can learn through chi gung practice, their are also many physical benefits to chi gung practice.    Relaxed muscle structure is one of the most common and immediate benefits (though the level of relaxation can increase practically indefiantely).  Relaxed muscle structure will increase speed, power (learning to drop the energy into strikes), iron shirt, and allow a practioner to move comfortably through awkward postions (bagwa and grappling are great examples of this).     Lengthening and strenghting of ligaments and connective tissue.  This can reduce damage to the joints (through training or attacks).