In essence, it is a heated bath that can be used both to cook food and to keep food warm over time. The term itself has become more loosely used to describe the type of pan used. So while its origins are focused on cooking and heating, modern bain maries are also commonly used to hold cold food items as well.

A bain-marie also known as a water bath or double boiler), a type of heated bath, is a piece of equipment used in science, industry, and cooking to heat materials gently or to keep materials warm over a period of time. A bain-marie is also used to melt ingredients for cooking.A bain-marie is a simple technique used for baking delicate dishes like custards, cheesecakes, mousses and terrines. It works to prevent cracking or curdling by surrounding the food being baked with hot water to produce an even, gentle heat.

The purpose of a bain-marie is to cold the food  if you want to store sauces, fresh vegetables. For custards and terrines, a bain-marie cannot be beaten, as they turn out very well. It's used for cooking delicate foods like custards and terrines to create a gentle and uniform cooling around the food.