As you keep using the Raspberry Pi, the SD card image becomes bloated and filled up with files that you do not need anymore. If you want to make a backup image or just want to have some of that precious SD card space back, it becomes very difficult to reclaim any of the wasted space.
One of the easiest way to get this space back is to do an apt-clean.
What apt-clean does, is that it clears out the local copies of the repository of packages retrieved for installation. So the older your SD card is and the more you have been using it, the better the chances are of recovering space back.
We have recovered as much as 400 MB this way.
Hope that this saves someone’s day like it did ours.