Why do farmers vasectomise bulls?

Vasectomising bulls make them into gomer bulls, which help the producer better detect which cows are in heat so that they, the cows, can be artificially inseminated.

And for your information, vasectomies on bulls are carried out by the veterinarian, not the farmer, and only on a bull or two that is too old to suffer the stress of having his testicles removed and can be used as a heat (or estrus) detector.