Best alternative for transactional emails with PHP Mailer, mail() function and Postfix

For a simple contact form and transactional emails I intend to use Amazon SES with PHP.

Researching I came across the following alternatives:

1. Using PHP Mailer class with the credentials of the Amazon SES.
2. Set up Postfix as Amazon SES relay to use PHP mail() function.
3. Using PHP Mailer with Postfix as the Amazon SES relay.

Questions:

1. Why hardly anyone uses the PHP Mailer SMTP connection directly with Amazon SES? Not a good practice? It is not scalable?
2. Why most professionals use Postfix as SES relay?
3. In some cases, why even using Postfix as a SES relay, still using PHP Mailer class?
4. What better and more professional alternative?