Mostly Business letters are sent with a specific subject and audience in mind but are not always clear with their purpose of writing a letter purpose.
To understand and obviously write a better letter.
Know why you are sending the letter, define its purpose -
Is the letter to inform?
Is it to request information?
Is it to offer congratulations?
Is it to get the recipient to act on a request?
All of these are very different purposes. You have probably received and sent many letters that, after reading it, left you confused because you didn't knew or could make out exactly what it said or had to say. The purpose were not clear in the letter.
Defining purpose while writing a letter helps the reader with clarity, and somehow it guarantees the letter to be responded.