A、To show special courtesy. B、To make themselves understood. C、To show their art of language. D、To show their superiority. A

In all languages there are different levels of formality and informality in talking to other people, The language and speech forms you use in talking to family members and friends are not the same as those you use in talking to strangers. The level of language you use in talking to strangers is more formal and usually contains more polite forms. In talking with strangers in English, people sometimes use extra-polite forms to show special courtesy.
    In a dialogue, for example, someone says, "Would you be good enough to mm the radio down a bit7" (Instead of "Turn the radio down, please. ") If you used this extra-polite form with a member of your family or with a close friend, it would seem very peculiar! However, when you use an extra-polite expression like "Would you be good enough’"" or "Would you be so kind as to…" with a stranger, you are trying to be very polite and courteous.