Most martial arts, weather they know it or not, teach their practitioners to protect their Centerline. Many cannot quite explain what a centerline is, explaining that it is an imaginary line down the center of your body. Not quite. The centerline is the carrying part, the structure of your skeleton that allows you to move upright. Defending the centerline means protecting the head, neck, spine, and legs from direct attack.
The Centerline is in two part, body, and legs. Each of these needs a totally different set of skill for protection. Hands and arms protect the upper body but should not be used to protect the legs. Legs need to manage their own protection by either moving out of the way, substituting a different part of the leg, or preempting an impact. This part is about the upper body and the use of hands and arms to protect you bodies centerline.
From the 'Vital Organs', only the brain had a full protection exoskeleton. The Heart, and Lungs are fully protected by the rib cage which also partial protects the Liver, Kidney, and Stomach (which can also have a solid layer of muscle protection). Intestines, throat, and bladder have no bone protection and very little muscles around to protect them. The most damaging attack on these organs are the ones that target a specific organ towards the spine or to where the organ is close to the surface like the Throat, Liver, Kidney, Bladder. Yet, the ones that need the most protection are the Throat, Solar Plexus and Groin.
Any martial art that did not protect these vital areas would not survive in the real world. Sports and Show martial arts aside, the first aim of any martial art is to protect. Yes, there are killing arts used by governments, organizations, and army's but these are also useless unless the person is safe. So, when it comes to a face to face confrontation having a reliable and familiar guard is essential. But that does not mean that that is the guard you use first when confronted!
Every guard is useful depending on the style and situation. Most common is the Boxer Style Guard that most realistic martial arts use. Sports style will vary their guards according to competition rules and show martial arts will not necessarily use a very practical guard. Yet, do not dismiss any guard until you have experimented with it. Everyone should at least have three guards and not rely on just one. An encounter guard, a fighting guard, and an o shit, my guard isn't working guard. Play around with the guards and discover which is best for your way and body type..