Preservative

Because vinegar contains acetic acid, it is perfectly suited to preserve foods. It works because most of microorganisms responsible to destroying food cannot live in an acetic acid environment. Preservation is probably the most common reason vinegar is used in food preparation.

Texturizer

Because the acetic acid in denatures protein it changes the texture of many foods that contain protein. For example it will tenderize meat.

Vinegar is also used to make pie crusts more flakey.

Because the vinegar will draw water out of vegetables, it will soften leafy vegetable. That is why it is a good idea to add the vinegar to a salad you want to remain crisp just before eating.

Acidifier

Changing the ph of foods can have many effects on foods. For example it can make yeast more active in bread making. It can also make baking powder more active.

Flavor Carrier

Many compounds that create what we perceive as flavor are dissolved in vinegar. This makes vinegar very useful for carrying various flavors. For example we can add various herbs and spices to vinegar and then add the flavored vinegar to salads, pickles, marinades, or other foods.

Sour Background Note

We have only five tastes; sweet, sour, salty, bitter and umami. Everything else we think of as taste, e.g. chocolate, raspberry, banana, etc. happens in the nose. But the brain needs helps to create the other "tastes". Vinegar is one of the food additives that works very well as a "palette" for other "tastes".