Use of "Mounted /system" with secret key combinations in recovery; 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 SYSTEM or 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. 

The mount /system in the stock recovery allows to mount the system for some reason. I'm sure it has to do something with debugging.