Comfort. Not worrying about concealing makes for a cooler and less constricting wardrobe.
Speed. It's faster to deploy if needed.
Deterrence. Most criminals will pick easier targets, never knowing if the armed person they see is the only one around.
Education. It's good for people to see normal folks exercising their rights.
Political statement. It does no harm for the powers that be to see at least a few armed citizens, and wonder how many more are around.
These are excellent reasons. For me, MOST of the time I CC. When I OC, it's for convenience. It's much easier for me to OC during the week, back and forth to work. I carry my Fobus paddle, so I can quickly remove it and hide it in my toolbox, and I can quickly put it back on as I'm leaving work.
I've tried with my MTAC, and it's rather difficult. Weekends, I always CC.