1. People on their way to the range - which is occasionally me.
2. People whose work requires them to carry. There is a pawn shop in uber-liberal Tempe where employees have to handle precious metals all day. The staff carries openly in the store and on the street when they go out to get lunch. Nobody faints. Anti-Preemption laws are a good thing.
3. Accidentally. Shortly after I moved here I was at Walmart and a rather breathless lady with an upper Midwest accent approached me (why me?) and said "There's a man with a gun in the next aisle!" I carefully poked my head around the end-cap and saw a 30's something guy fussing with an infant who was side-addle in a car seat in the the guy's shopping cart. He was voicing his displeasure at the overall state of the world, or maybe he just needed a change. His 2-ish older sister was held in dads right arm, her arms around his neck and looking away in disgust at the behavior of her younger sibling. Mr. Mom's golf shirt had ridden up and the little girl's foot was resting comfortably on the butt of daddy's four o'clock Glock. I tried to comfort the panicked snow bird, telling her that the guy didn't look too dangerous and besides, if he tried to cause any trouble there was probably someone in this aisle with a gun too. In some places Mr. Mom's exposing his firearm like that would have been a criminal offense, even if he had a state license to carry it concealed. I'm glad I live in a tolerant, civilized place.
I open carry in Arizona quite a bit, and I see some others open carrying.
As a fourth distinct case, I saw lots of open carriers with rifles, pistols, and shotguns protesting against President Obama’s attack on the Second Amendment.