Why customization of /etc/postfix/main.cf and /etc/postfix/master.cf gets reverted?

Question

Customized entries in
/etc/postfix/main.cf or
/etc/postfix/master.cf get reverted after the command
plesk repair mail
was run, Plesk microupdates were installed, or after changing settings in Tools & Settings > Mail Server Settings. Why does it get reverted?

Answer

Intended behavior. Plesk re-writes the config file every time mail settings are changed.

A vote can be left on Plesk UserVoice regarding the feature of keeping customizations untouched by Plesk: 
Allow customization of /etc/postfix/main.cf.

Allow customization of /etc/postfix/master.cf.

The top-ranked suggestions are likely to be included in the next versions of Plesk.