I’m in the UK, and where I live, it’s almost exclusively local council owned toilets that charge a fee. So these aren’t toilets inside private businesses, they’re separate buildings located in car parks, at beaches, and so on. So the fee to use them is almost certainly a combination of preventing homeless people from squatting in them (since they’re not watched over by staff) and to cover the costs of electricity, water, and sending someone over to clean them once in a while (since the majority of people using them are not residents of the area who have paid council tax). The fee is nominal, £0.20, and most of them now have card readers so people don’t need to have a 20p coin on them.