Night flying is necessary for certain crops and in certain locations. The reasons why this is true range from protection of benificial insects (bees, for example; spraying during times when the bees are not in the fields is necessary). Lower wind conditions, crop requirements, chemical labeling requirements, lower temperatures (volatility, better flying), etc. Some pilots agree they can spot powerlines and obstacles better at night, others don't.
If your question is whether a pilot getting into ag work will be required to fly at night, the answer is no. You can easily go your entire ag career and never fly at night, as ag work is primarily a daytime occupation. it's not uncommon to start prior to sun-up and end after sun down, but lots of guys go years and never see a night landing. For an entry level pilot, jobs which require night flying are generally out of your depth anyway, and you won't be asked to do it.
You can always look for the jobs that don't require it. They're in the majority.