If members of your family like having second helpings at dinner, it's so much nicer if gravy or hot sauces are still hot. And, if you rinse the vacuum flask out with hot water first, and then pour in your gravy or sauce at over 70 degrees temperature, it stands a better chance of not multiplying any of the spore forming type of bacteria (the others will have been killed by heating) by being kept hot, rather than cooling in a gravy jug.
Making custard before serving the main course, and keeping it hot in a thermos flask also saves all the arguments about who is or is not going to eat the skin that forms when custard is kept for any period of time in a wide mouthed bowl or jug.
