Answer:
To prevent heat loss.
Step-by-step explanation:
Vacuum flask is a container formed by two bottles one inside the other, joined by the neck. It works as a thermal insulator.
It can be filled by hot or cold substances while maintaining its temperature for a long time. The silver layer found in the inner flask acts reflecting the heat radiation. This causes most of the heat to be reflected back inside or if it comes from outside, to be reflected out.
The space between the jars has no air. That's where the name vacuum flask comes from. This is so that heat is not transmitted by convection or conduction.