Postfix is a mail transfer agent, it's used to send email from within your computer system (email is inherently built into every unix-like operating system). This is sometimes used to record notifications for system administrators (i.e. an email to the 'root' user account) and things like that.
In short, you can safely select
No Configuration. You absolutely don't need to worry about it.
The reason why postfix was configured was that
bsd-mailx depends on it, which in turn is a dependency of
devscripts, which is installed as part of the debian build system.