In a gunfight, you never want to stand out in the open, like high noon in an Old Western movie, and shoot it out in a fair fight. There is no such thing. You had better believe that if a bad person is shooting at you, they are going to strive to make it as unfair to you as possible and you should return the favor. One of the best guides in your tactical toolbox is the proper use of cover and concealment.
What is Good Cover?
Cover is something bullet resistant that you can tuck most of the important parts of your body behind to ward off incoming bullets. This structure needs to be durable to be granted the term cover as bullets can and often will penetrate much deeper than Hollywood would lead you to believe. Think of adequate cover as thick, durable barriers like boulders, vehicle engine blocks, brick walls, and retired battleships. Freestanding cinderblocks have shown to be blown through by 9mm FMJ rounds (although if properly filled with cement this seems unlikely). Keep in mind that a sheetrock wall is not cover as this simple wallboard can be penetrated by even .22LR ammunition with ease.
Some cover is not all-inclusive but is better than nothing is. For instance if you are caught in gunplay in a suburban environment, laying prone behind a high street curb will provide better cover than standing in the open. If all else fails, getting as close to the earth as possible can reduce the possibility of getting a stray round parting your hair. As a combat veteran once told me after being caught in a nasty three-way firefight in Indochina, hugging the ground until you leave button prints in the dirt often works.
What is Concealment?
(As seen above, a roll up garage door is more a case of concealment rather than cover)
Concealment is not necessarily cover. As its name implies, it simply hides you from the range of view of the person sending rounds your way. This can be a building edge, automobile, or bus stop if outside or a couch, kitchen counter, desk, or large chair if inside structure. While most of these offer almost no ballistic protection and thus do not provide cover, they do hopefully keep you from having fire directed at you. This is a passive form of self-defense. Think hide-and-seek but you get a round in the face if you are tagged.
Some concealment is also cover, but not all cover is concealment.
Why are they needed?
(Be sure to train from the barricade regularly)
(Outside the home, the possibility of making shots from the cover/concealment of a vehicle should be kept in mind if things go pear shaped. Needless to say, this should be practiced even if its via dryfiring of a safe and clear weapon)
While it is not probable that you will walk into or wake up in the middle of an active gunfight, possibly with you as one of the shooters, it is possible for it to occur. At the first indication of gunplay, it's best to react to this, whether armed or unarmed, by looking for the best and closest cover. If cover is not available, concealment is a close second. If you are unarmed stay there and wait to break cover until the situation gets a little more tranquil. If you are armed and it's possible to engage, do so. As such practicing from a barricade is an absolute must in any regular training.
Either way, always remember that incoming fire always has the right of way.