As the bacteria that spoil food aren't necessarily the same ones that cause food poisoning, if you rely on smell and taste to override the maker's use by date, you are taking a chance. The idea is that if the product is stored properly, any initially low levels of bacteria won't grow dangerously or produce toxins before the use by date. The maker will be cautious when setting the date, so there's usually a safety margin, but there's no way to know how long it is. Usually food will be safe after the use by date, but no guarantee, and quite likely no redress.
But some makers are confused about use by and best before dates, some cheat, some don't know how to set the date and guess, and some just copy what the others do. Then there are caterers who ignore the dates, retailers who conceal them, and consumers who take a chance, like with the cream, and all of them might store the product incorrectly. So the dates aren't totally reliable. And not many people who keep left-overs put use by dates on them.
But some makers are confused about use by and best before dates, some cheat, some don't know how to set the date and guess, and some just copy what the others do. Then there are caterers who ignore the dates, retailers who conceal them, and consumers who take a chance, like with the cream, and all of them might store the product incorrectly. So the dates aren't totally reliable. And not many people who keep left-overs put use by dates on them.