Answer:
A vacuum flask is usually used to insulate it's contents… I've used them for a multitude of things ranging from keeping my coffee warm, to keeping liquid nitrogen from evaporating.
The reason these types of flasks are so efficient has to do with their thermodynamic properties. To understand how a vacuum flask works, we need to know the basic properties of a vacuum. Air just like any other molecule is a conductor of heat.
Firstly, a “layer of vacuum” is created around the thermos, it's crucial that the chamber is depressurized*.