I started to OC for a couple reasons. I got my first handgun when I was 18 or 19. Not being 21 yet, OC was my only option. I started actually doing it because at the time I didn't live in the best area and felt it was necessary to carry a firearm on a regular basis for self-defense.
Now, I still choose to OC over CC mostly because it is more comfortable and I think it is a good way to educate the public about guns and that not everyone who carries a gun is a criminal.