Importance means that something is significant, while an advantage is something that helps someone or puts someone in a higher position than someone else. Something can be important without being advantageous. Important can also have different meanings for different people. You may think that it is important to have good morals, but this will not necessarily earn you more money, which would be an advantage. Hope this helps.
The salutation is to greet the person you are addressing, it goes before the introduction. The introduction is a brief summary of the letter itself and the body is the letter itself.
