Restaurant sauces are there not only to provide moisture but also to complement the flavors of whatever they are put on. For a chef, this means more than just mindlessly putting tomato sauce on spaghetti or chocolate sauce on ice cream.
Pairing a sauce with food takes a bit of analysis and thought to get it right. A chef sauces food to show it off at its best by making sure that the flavors of the food and sauce are well balanced.
A rich fatty fish benefits from a tart lemon sauce. The sweetness of pork or venison can be underscored by serving it with a fruit-based sauce.
A juicy, fatty prime rib is perfectly complemented by a simple au jus while a very rich sauce can be too much of a good thing. Like a chef, try to identify the main flavor and texture characteristics of the food you are serving and then pair a sauce with it that will present a nice contrast of flavors and a balance of flavor.