AFAIK, you only need to use it in HAM mode if you want to use licensed frequencies, a higher power transmit (assuming the radio supports it; US 915 can transmit up to 1W/30db unlicensed and many radios can only transmit at 22db max), or to go beyond the airtime limitations (there no limitations on airtime for US 915). HAM mode also disables encryption if I recall. Also AFAIK, you're not required to use HAM mode just because you are a licensed HAM operator.
Sources: Have read the docs but am not a licensed HAM.