Benefits are less load on the battery while loaded up like headlights on full, heater blower on full, rear heated screen on especially this time of the year. Getting your battery upto full charge quicker good for people that use the car for short runs or not regular.
It wouldn't over charge the battery the alternator has built in regulation and knows when its under load "supply's to demand". Pulley size is usually the same on the Same model of alternator it just has higher output (thicker winding on the stator and rotor plus bigger diodes in the rectifier)
Usually the alternator gets upgraded when you add more electrical load like a huge sound system or running an inverter in a campervan, towing a caravan with a leisure battery or fridge thats on while the caravan is being towed to destination or the likes of a vehicle that run a refrigerated unit or electric heaters.