Pretend you’re in a hot tub and you ask to turn the bubbles on and only a few come out, you sit there disappointed. Well, that’s how pasta feels when you simmer it instead of boil it in water. Boiling helps keep food in motion to prevent sticking, and to cook something quickly. It also helps in retaining flavor and color in vegetables (the process is known as blanching). When you want to reduce something, such as sauce, you boil, as it causes speedy evaporation. Boiling comes in handy for grains, rice, pasta, vegetables and when you want to reduce something. Add pasta or vegetables to the pot once it’s actually began to boil rapidly.
Simmering on the other hand, cooks food slowly and gently. Foods that do better in simmering liquid are meats (to keep it moist and tender. Boiling toughens proteins in the meat and will cause it to be dry and tough), stocks/broths, Fish, poultry, potatoes, turnips and beets.