Any time you want something and wish to say it politely. It would certainly be used in restaurants. Waiter: "What is your order, please?" You: "I'd like the soup, followed by the spaghetti, please." In those situations, you could also say "I'll have..." You can also use the phrase to indicate that you want something, again, in a polite manner. Saying "I want.." is considered too direct when asking someone for something.