It's especially important for defenseman. You want the dominant hand on top so you can poke check with your better hand. For some reason, it's mostly Americans that tend to play opposite of that. I'm guessing it comes from playing baseball, where hitters swing with the dominant hand further up the bat.
Maybe more simple than that in some cases, especially if somebody grew up in a non-hockey area like I did. If my clueless parents bought me a stick as a kid, they'd probably have gone to a sporting goods shop and had a random employee ask if I was right or left handed.
I did teach myself how to switch hit when I was a kid since my favorite player was Chili Davis. I'm now equally mediocre hitting right or left handed. Although it was always fun at company softball to take my first AB left handed. Then when I came up the second time, the defense would shift as I was walking up to the plate and then I'd take the AB right handed.