[pjc50]

Dependencies in debian systems which rely on "virtual" functionality can often have surprising results. At a guess removing cron removes things which depend on it - perhaps one of those is Exim. Something else "requires" an MTA, so it installs postfix instead to fill the gap.

[ThatGeoGuy]

This would be apt. Apt requires an MTA because maintainer messages / change-lists are sent as mail to the root user.

[stevekemp]

Close. It is cron that requires a package which provides `mail-transport-agent`, because output from cronjobs is traditionally emailed to users.

You can see this via:

      apt-cache show cron|grep -i mail

Note that the `apt` package itself does not require a MTA.