About 10% of the population is left-handed. Being left-handed affects daily life in some way that are not obvious, with many devices being designed for right-handed people. It can also affect participation in some sports, sometimes being an advantage and sometimes causing an disadvantage.
An advantage for left-handers in martial arts, and some confrontational sports like boxing, is that most competitors are used to fighting right handers. A left handed opponent will often move in the mirror reverse way to what is expected, leading with a different foot or hand. This can be unexpected and slightly awkward to fight against. Even other left-handed fighters find a left-handed opponent takes some getting used to. Occasionally a right-handed individual will train to box left handed because they believe there is an advantage to be had. In martial arts this advantage does not appear to be pronounced as it is with boxing.
Really, to be effective fighters we should learn to use either side of our bodies, blocking and striking with either hand as the situation requires. The advantage of using the opposite approach to what our opponent expects will not hinder us if we are used to fighting equally well with either hand and foot.
The gap between left and right handed use may partially close in modern society. Typing on a computer is becoming more common than writing with a pen, and typing requires the use of both hands. Over time we tend to develop some proficiency with the previously weaker hand. Musical instruments and some video games further this trend. Martial arts training may both benefit from and contribute to the development of using our left and right sides.