When you S-Off you are allowed to change more of your system than if you had S-On.
You can see it clearly when you boot into the boot loader. You can do this by turning off fast boot in settings > power. Then shutting off your phone and then hold power and volume down.
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Because with a ROOT and a custom recovery I can do basically anything I want...
What is really useful for? For example... I got a G3 and all I need is to unlock boot loader, root, flash custom recovery and then I'm free to do what I want. Now with the M8, that I'm helping a friend to unlock boot loader and all that, I see that S-ON/OFF and don't see what is it's importance. Can you give me some examples of changes that I can do with that, that it won't be able with a simple flash via custom recovery?
Thanks in advance!