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.
Could be. As is said so often: physics doesn't scale. Trailing edge eddies may not be the major source of the noise in this anyway - since these are pushing air not being pushed by it.
They are surrounding a farm with hunners of cows in multiple fields, so I wonder if the noise is quite scary for them, hence efforts to make them quieter. Or the farmer insisting. Are they usually quite loud?
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.