I recently destroyed the system partition on my Ubuntu 14.04 machine. I hadn't bothered to back it up since I knew I could easily restore it from a DVD. The problem is that every once in a while I come across a package that I need that I don't have. Of course, I can install it easily with apt-get, but that interrupts my work flow.
It's too late for this machine, of course, but in the future, I would like to have a list of all the packages installed on the machine. Then I could do something clever like
xargs < file_with_list_of_pckgs | apt-get install
Then periodically, I can create a list of packages. If I ever zap the system partition again, I can install Ubuntu from DVD and then use apt-get to get the packages I am missing.
I also have to backup everything under /etc.