1.   To document a time

 It was 8:34 A.M. when John finally arrived at the meeting!

2.   To introduce a list

More than knowing when to say "please" and "thank you," formal dinner party manners involve other factors: dressing according to the level of formality; arriving on time; contributing a beverage, meal, or decoration like a bouquet of flowers (or, at the very least, offering to!); waiting for the host and/or hostess to be seated before beginning to eat; using the appropriate silverware during the appropriate course; and sending a thank you note afterwards.

      Note: The list that follows is not a complete sentence.  Unlike semicolons, which are often used to separate complete sentences, colons should not have complete sentences on both sides of them when used in this context.