Before you can fully appreciate the mother sauces, you should know what a sauce is. Sauces are thickened liquids used to add richness, flavor and moisture to a dish. Drier foods – such as grilled meats, roasts or meatloaf – are often enhanced with sauces and gravies.
Sauces typically contain a liquid, thickener and various flavorings and seasonings. French mother sauces use milk (béchamel sauce), white stock (veloute), brown stock (Espagnole), clarified butter (Hollandaise), and tomato (tomato sauce) as the liquid base for each type of sauce.
Enhances flavors.
Some sauces help in digestion, e.g. mint sauce, apple sauce with roast pork.
It gives moistness to the food, e.g. white sauce adds creaminess to firm and dry food.
Adds color to the food. Hollandaise sauce served on a vegetable adds color.
Served as an accompaniment, sometimes gives a contrast taste to another food, e.g cranberry sauce with roast turkey.
Sometimes gives the name to the dish ,e.g Madeira wine when added to the brown sauce, it is called sauce Madeira.
Enhances nutritional value of the dish.
Dresses and complements food that need some additional quality and makes the food more palatable, e.g. Chaudfroid sauce is used to coat various food items, and gives a good appearance.
Gives tartness and contrasts or balances a bland food, e.g. Devil sauce served with eggs give appealing tartness.
https://www.everydayhealth.com/diet-nutrition/what-are-5-basic-mother-sauces/
https://foodsgoogle.wordpress.com/2010/05/30/importance-of-sauces-in-food-preparation/