Why do you blanch?

Answer 1

When blanching food, you cook it partially. This insure that vegetables does not go soggy, stay crunchy, food retains it colour (especially green vegetables) and over cooking certain food can make it tough, for instance squid. Restaurants blanch the vegetables before service and then dip it in ice water. The ice water stopping the cooking process. When a portion of vegetables is needed, it is sauteed in a pan, this saves cooking time when the orders need to go out.

Answer 2

Blanching is done for various reasons.

1. To cook something very lightly then stop it from cooking further in its own heat.

2. To destroy enzymes which would affect the quality of the food if it's to be frozen.

3. To soften the food (e.g. the skins of nuts)

There is a more comprehensive answer on wikipedia.