When and Why do you use three quotation marks?

Double quotation marks " are used for a quote:

John said, "Hello there." and quickly walked away.

Single quotation marks ' are used for quotes within a quote:

John said, "Thomas Jefferson once wrote, 'All men are created equal.' and it's still true today."

Triple quotation marks '" are used when the quote within the quote falls at the end of the sentence:

John said, "Thomas Jefferson once wrote, 'All men are created equal.'"