Why debian use postinst for declarative actions?

[George Shuklin <george.shuklin@gmail.com>]

Hello.

There is a big question with torture me for awhile. Why some purely 
declarative operations are performed by non-standard postinst scripts? 
I've checked few well-established packages (openssh, nginx, systemd, 
dbus, cups, etc) - each of them have slightly different code for such 
operations.

Example of such operations:

- adding and removing users
- changing permissions and mode for files
- creating symlinks
- reloading/restarting daemons

etc.

Is there any kown reason why such operations are forced to be 'write it 
again and again' style, or it's just a historical luggage?