Left-handedness is a disadvantage in society where everything is designed for the right-handed person. However, there is one advantage to being left-handed: in hand to hand or sword to sword combat, the southpaw has an advantage in being unfamiliar.
This article http://www.economist.com/node/3471297 (Economist, 2004) describes an empirical finding: in societies that were historically more violent, the left-handedness proportion is higher.
Perhaps this is also why left-handed people even exist. They have an advantage in historical person-to-person violent conflicts. This advantage would go away as their proportion gets higher. I presume (this is conjecture) that the minority proportion that it is, is the steady state equilibrium where the violent advantage outweights the other disadvantages of being out of sync with society.
Their article on left handedness goes into great detail as to the biology behind hand preference. In short, it's related to brain patterning and the direction the hair on your head swirls (seriously!): http://omim.org/entry/139900
As for the evolutionary reason, the most recent research suggests that it may have to do with cooperation. Simply put, it's easier to cooperate when everyone (or most everyone) uses the same hand for most activities. More details here: http://www.northwestern.edu/newscenter/stories/2012/04/left-...
Answer from an individual that studies "handedness".
My uncle was born right-handed. When he was young, he broke his collarbone on the right side. While this was healing, he used his left hand, and continued to do so from then on.
Then, working on farm machinery, he ran his left hand through a V-belt, injuring it badly. While that was healing, he went back to using his right hand again.
Probably not very typical.