Just that, really. I don't. But was wondering whether it would be useful? Could there be a scenario where bank cards would stop working and you'd only be able to buy things with cash for a few days (e.g. petrol, milk...). What if there was an economic collapse - would it be any help to have a couple of hundred at home, if the banks shut?
Likewise, in the car? I've got a tenner in there, but not sure what I'd use it for! Don't want to keep much more in case of theft.
I suspect what would happen would be I'd have a 'secret' cash envelope but keep snaffling notes from it for my wallet when I run out instead of using the cash machine, and then when the crisis hit the envelope would be empty!