Generally speaking I get a pretty large sum of cash out at the beginning of a trip upon arrival and try to use cash for everything. It does depend on where I'm visiting, but the reality is that in a lot of the world there are places which only accept cash or where you can have a simpler transaction using cash. When I return, I just dump whatever remains in the jar. In theory I would take it with me if I visit a place that uses that currency again, but I usually forget and since I have to get more from the ATM anyway I don't really worry about it. It's not as if I brought back 300 euros from one trip. I have currency in there right now from about 28 different countries.

Regarding cash to carry, I basically am not concerned with pick pocket / theft, for a number of reasons, and am more concerned with a business being unable to take a card for some reason. For instance, my primary travel card is an American Express which is not accepted by small businesses in a lot of countries, such as restaurants and shops. In these cases I have a backup card, but I would prefer to use cash. I also ensure I have enough cash to always get me back home and in communication, so enough to replace my phone and get a return plane ticket.

I imagine locals don't need to carry as much cash as foreigners because they have a different set of concerns and easy / ready access to more cash without being charged exorbitant fees for access.