If you on the other hand want to flash a custom ROM build for a higher firmware than the one you're running s-off is needed to flash the firmware corresponding to the custom ROM you like.
Sent from my HTC One_M8 using XDA Free mobile app
Mobile: One M8 Gunmetal: UNLOCK - S-OFF (SunShine) - WWE
CID: HTC__001 (T-MOB101)
FW: 2.22.401.5 (22.214.171.124)
ROM: Stock - SuperSU
Retired: Desire (CWM, Cyanogen), Sensation (Unlocked, S-On, CWM, root), One X (Unlocked, TWRP, Cyanogen), One (M7) (firewater S-Off, TWRP, ViperOne), Moto G (Unlocked, S-On, CWM, CM11), One (M8) GER Silver (SunShine S-Off, TWRP, ViperOne 2.2.0).