But my main concern is why shoule we have DB2 back up when we have system back up which have almost everyting in the system?
because the process by which the relative backups are taken are different and because
usually the <system> backups concerns only the software, not the application <data>
but as usually all depends on Your organization standards
also in large organizations the responsibilities for different environments are spread between different groups
and each group is supposed to take care only of its <things>