One is simply not carrying something obviously containing expensive electronics which might be a target for theft. But having thought about this for a while that's not my biggest motivation. I hate ostentation. That's the reason I wear a Casio watch, would never wear clothes with a brand name plastered all over them, and drive a very boring car.
I do have a laptop sleeve that I use to protect the laptop and it slips easily into a bag. Prior to my silent bag purchase I did use random shopping bags (including canvas bags that seem to have sprouted like weeds in my cupboard), but after the board meeting situation mentioned I decided I'd try to find a long-lasting "paper" bag for the hell of it.
And I stumbled upon movie prop bags! Oh, and I hate backpacks.