S-off isn't needed to:
- Flash TWRP
- To achieve root
- Flash ROMs
- Needed to flash radios (SuperCID also needed to flash a radio that does not match your version)
- Needed to SIM unlock
- Needed to flash modified hboot (such as to remove the red warning text form boot screen)
- Needed to flash firmware components (not the ROM)
- Allows you to run any RUU (other carriers, or even previous RUUs, which is not normally allowed by version check)
The last one is important, since s-off essentially gives you more recovery options if things go south.
But s-off means all security checks are off by definition. This even includes device check, which means flashing a ROM or mod for another device is possible. I've seen folks flash a ROM for a different device with s-off, and brick their phones (due to different partitioning, etc.). As long as you are very careful to only flash things intended for our device, you will be fine. But anyone with s-off (or interested in s-off) needs to understand this.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)