It's often a good idea to make the root file system in a embedded system read-only. If you do this and only make changes to files in a ram disk (mounted under /tmp, for example) the device will always come back to a known state when powered up. This is a nice feature and often a requirement.
But sometimes you need to make changes which persist. Check out the "mini fan out" file system. It allows you to layer changes on top of a read-only file system.
I have not used this package directly, but I plan to shortly.