can fastboot be used to backup device? Many people tell about fastboot that it can be used to flash a image on one of the partitions.

Is the reverse also possible, that is take a binary snapshot and store it as a zip file containig the exact binary representation of the partition to be backuped?

the reason I ask this is
backup has two steps.

1) take and image/snapshot to backuplocation (backup)
2) and replacing image/snapshop from backuplocaito (restore)

to me it seems that fastboot is only doing one, while being able to write to the Androids internal flash (in binary "dd"-style) seems it should also be able to read from the flash.

This way nobody would need to organize and run another potentially not trustworthy rom to do the reading job.
If you check with our fastboot tag-wiki, and follow up the link to the List of fastboot commands, you will see the answer is NO. Fastboot only has commands to write to the device. A few exceptions include:

making sure there is a device at all, using fastboot devices
some OEM specific commands to read configuration values (fastboot oem &lt;option&gt; – note that all those, except for oem lock/oem unlock, options start with INFO?)
several bootloader specific commands
rebooting the device after &quot;work done&quot; (fastboot reboot / fastboot reboot-bootloader)

From the CyanogenMod Fastboot Intro, emphasis mine:

fastboot is a small tool that comes with the Android SDK (software developer kit) that can be used to re-flash partitions on your device. It is an alternative to the recovery mode for doing installations and updates.

Wikipedia describes fastboot as a diagnostic protocol included with the SDK package used primarily to modify the flash filesystem via a USB connection from host computer. (again, emphasis mine).
No source speaks about backing up partitions (or data) from the device.