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.
this post was submitted on 16 Jun 2023
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Same reason you see chevrons on the back of some modern turbofan engines
Helps them chop up birds better
As others have said, mostly about noise reduction. There's actually work looking whether the same concept could be developed for electric aircraft.
They disrupt the trailing edge eddies - which is what causes any noise - and so make them quieter. They are often fitted in areas where sound might be a particular issue.
Trailing edge you say? So not designed to mince up birds, that's a relief.
Oh they’ll still mince birds, just with the blunt side
Most of the bird deaths are not actually physical strikes - it is a result of the massive change of air pressure as the blades pass at speed. Evidently painting one of the blades black - or some contrasting colour - keeps a lot of the birds well away.
So why don't my CPU/case fans have serrated edges as well? Because they are too small?