-Left/right balance (kinda, see below)
-Many Shimano road cranks can’t read power from the (edit: drive side) crank accurately
-Some frames and cranks don’t have enough clearance
-Many SRAM MTB cranks can’t use a crank PM due to no flat surface for mounting
-Not buying another crank arm (or altering an existing one)
-Generally longer battery life and rechargeable (see many issues have with coin cells)
-Probably More aero