Unlocked bootloader gives write access to the system partition which s-on would protect if it was mounted read only.
S-off disables the protection feature including being able to unlock your phone from your service provider ( if it happens to be).
I just needed write access to system to make updates easier (adaway, ad-hoc enabler, etc)
Sent from my GT-P7510 using XDA
I'm now using S-ON Unlocked and AdAway didn't crash so it's working for me now.
The sole reason I wanted to root now works.
2 years wait but finally it works, I can now store my alarms, ringtones on the phone and don't have to worry about using USB diskdrive without losing the ringtones since the SD card is in use.