Unlocking the bootloader is just turning off the security so you can modify and change the first on your device.
Unlocking the bootloader is required to root or flash a custom rom.
Unlocking the bootloader is the first step to rooting but just unlocking the bootloader will not give you root. You need to flash the su patch or flash a custom rom to have root.
S-off is getting the security off of the radio, hboot, and kernel so you can change and modify them.
You only need s-off if you want to use a custom kernel, fix radio errors if you have them, or want to use a non sense based rom.
All aosp or gingerbread roms require s-off and a downgraded hboot.
Sent from my HTC Evo V 4G