Question

Is a business letter, what is the purpose of the heading?

Answer 1

The heading indicates where the letter originates and when it was written, it also contains the return address. Sometimes it is necessary to provide other means of communication such as a phone number, email etc. so they can get back to you faster, easier, and cheaper.
The heading is used as a way for them to contact you with any information etc. It is also important if the business letter is going to be put in a file(which many are) so that they can see when it was written and by who, so they know if it is still relevant to them. 

Answer 2

The heading of a business letter should contain the return address (usually two or three lines) followed by a line with the date. The heading is indented to the middle of the page in the modified block and semi-block styles. It begins at the left margin in the block style.