It reduces the turbulence at the trailing edge, reducing noise. Bernouli’s principle dictates that part of the force on the blade is due to the airflow on each side moving at different speeds, if the crashed into each other all at once, you’d get a lot of turbulence, and hence noise. The serrations ensure that the two flows meet over a longer distance, reducing said turbulence.
It reduces the turbulence at the trailing edge, reducing noise. Bernouli’s principle dictates that part of the force on the blade is due to the airflow on each side moving at different speeds, if the crashed into each other all at once, you’d get a lot of turbulence, and hence noise. The serrations ensure that the two flows meet over a longer distance, reducing said turbulence.
Same reason you see chevrons on the back of some modern turbofan engines