That you dont have to struggle switching the sail while laying in the water. This way, if you fall, you can get up and go sailing immediatly and dont waist energy in the water. That means more time on the board which means more progress.

As Philip said, you don't have to waste time to switch side of sail, waterstart immediatly. It will come handy when you'll start to sail in switch.

Yeah..you wont have the need of flipping the sail at normal position to do waterstart

Imagine for example, you are in the waves and you crash in the water, then the next swell is coming, so you need to do a quick water start to get away

as the others said, it comes in handy in nasty situations where you need to get up fast. you have plenty of time to switch the sail later

It makes it faster to get out of the water if the sail isn't in the correct position for a water start. Benefits with wave sailing, to get up faster and avoid swell while in the water

If you stack it midway through a gybe but keep hold of the rig, you can waterstart from the same position i.e. clew first!!

Going for a gybe and you mess up the exit but you never let go off the sail, you can just clew first waterstart straight away and keep going without hassle