Often, if a practice was already in place on a property when the ordinance that will forbid it changes, it is grandfathered in FOR THE CURRENT OWNER, but it typically will not be permitted on that property into the future when the property changes hands. Sometimes the "grandfathering in" may still apply with changes of ownership to a relative of the current owner.
It's a way of accomplishing the goal of the zoning change over time by attrition. "Grandfathered in" has it's + and - though too. Often, once a zoning change takes place, say when a town or city annexes land for expansion and "urban sprawl", and the land zoning changes from agricultural to i.e.: commercial..., the property value and taxes on the property will also drastically increase, to the point that the "ag" guy will no longer be able to afford to stay, and continue to float the tax burden from his farming operation revenues. So he'll end up selling off pieces of the farm, just to be able to continue and pay the taxes. He can no longer "expand" his operation to increase revenues... he'd never get the permits...zoning won't allow "expansion" outside of the current zoning restrictions... only "continuation as is" with what was there when the zoning changed. Eventually, all the hassle of trying to farm through all the traffic and development, and the complaints of neighbors just isn't worth it anymore, so he sells out. Yes, at that point, he's got some money in his pocket.... but he had to give up the "family farm dream"... Grandpa's farm... often a "fourth or fifth generation century farm" even, and if he's got enough years left in him, he has to leave that history behind, and start over again, farther out away from the city.
That's just the way it is. When my Mom was a little girl (1930's), her Dad was a minister. They lived in a small town, on a small town lot. But they had a little barn in the backyard, and they had a milk cow and about 30 chickens, for eggs and meat, and they had a pretty large garden, onto which they put their manure. Not many "towns" ordinances will allow that anymore. There's reasons for that. I live nearby to Rochester, MN, home of the Mayo Clinic. Even the Mayo's had a farm "in town" with cattle, and sheep, and horses... they were quite proud of them, and the Mayo Clinic is proud to display that image of the Mayo brothers. But Mayo Clinic is the biggest driver of growth in Rochester today, and that growth has drastically changed the "agricultural landscape" that Rochester and the surrounding communities used to be. The area is very hostile to agriculture, and ESPECIALLY animal agriculture, anywhere near to the city. They see our farms as "vacant land", and us farmers that operate it as "peasant tenants" that are just taking care of the land for them until they have need of it.
How does urban sprawl happen? A BIG contributor is when those who don't make their living off of the land, want to have their little piece of heaven in the country (can't blame anybody for wanting that!).............. but that then means you will have a non-ag residence in the ag protection district. Housing developments that consume 2-5-10 acres per household, typically in what was an ag protection district. That's an awful lot of land just for a house. Urban sprawl. Expensive "life savings" (your words) homes built there, and now they fear the "agriculture" next door might negatively impact it's value (and they're right... IT MAY!!!). They don't want to be smelling manure, hear cattle bawling, have dust, or hear tractors and combines and trucks running at all hours of the day and night, they don't even want to be able to hear a windmill turning a half mile away. They think that when they bought their place, that meant that they then "owned" the view across their neighbors land too, and because of that, they insist on dictating what their neighbor can and can't do with their land, because it'll "impact their view", or "impact the peaceful serenity" that they moved out into the country to find. Once these non-ag residences outnumber the actual farmers that live and make their living in the township, they then control the township, and overrule the "ag voice".
Sense a little hostility? You're right. The problem is, nobody thinks that THEY'RE the one that will cause these problems, until they ARE. THAT'S why we now have zoning ordinances............ and THAT'S why variances to those ordinances should not be granted easily.