A process used to blanch foods or re-hydrate dried vegetables while simmering is an method used for the extraction or fortification of flavors as in the process for making stocks and reducing sauces.

Pastas and rice are boiled rapidly so the exteriors gelatinize and prevents the products from sticking together.  

Simmering is the preferred method for meats as it is a more gentle process and will relax the tissues or break down connective tissues over a period of time.