I just discovered that after pressing a secret key combination in recovery mode, the log below shows a message "Mounted /system". What's this used for? Thank you!
mount option im recovery mode allows you to mount a partition , like
DATA etc. Like you would do with a computer drive. This allows you to modify anything in that partition. Because when you boot in recovery mode, the other partitions are not mounted, you cannot see data or edit them.
mount /system in the stock recovery allows to mount the system for some reason. I'm sure it has to do something with debugging.