There can be as many reasons as there are people doing it. It can vary from a small garden to a large grain farm to a livestock operation. To be certified, there are rules, guidelines, procedures and practices that are required or suggested.
For me the reasons have varied as time has passed. A person who has been a great influence on me was our chapter president and personal friend, Sam West. He, like most of us, used everything that everyone recommended in the way of chemicals and fertilizer. Then, according to some of the low rate fertilizer people, you do not need much inputs if your Calcium levels are adequate. So, Sam said to himself, if I can use a lot of high Calcium lime and very little fertilizer, then why use fertilizer at all? So that is what he did. And he told everyone who would listen.
I listened for a while. Then, circumstances dictated that I do something. We had farmed up to 1000 acres like everyone else in those days and were quickly going broke. Then my father was ready to retire, various landlords of rented ground were deceased, and all of a sudden I was down to my dad's home farm. So, as quickly as I could I applied High Calcium lime to every acre. It took 4 years to get the levels to where it should be. In our case, it required about 15 tons/acre. The agronomists and fertilizer people said "Oh my, you are ruining the soil and you will surely have micronutrient deficiencies." But I quit using chemicals and fertilizer. I found, like many others, that as the Calcium levels increased, other nutrients became more available. So I quit worrying about that. Now, I knew without the expense of a lot of inputs, I could afford to grow less bushels and still come out ahead. This is true. In fact, the limiting factor, other than what Mother Nature decides to dish out, seems to be competition with weeds. I had weeds. Sam West did not. So what was different? Sam seemed to be growing as good a yields as he ever had when using everything. The answer, I think, is that he still rotates his crops like most of our fathers and grandfathers did years ago.
So, I am now implementing a rotation- corn, soybeans, small grain and legumes. I have found already, that after a good stand of clover, there is very little weed competition. And if you follow the clover with corn, you get the advantage of the Nitrogen boost.
Now, I think maybe I have it figured out, Maybe. It is a whole combination of things that will allow you to farm successfully without chemicals and fertilizers. High Calcium Lime helps, crop rotation helps, moldboard plowing helps, and maybe growing a legume such as Clover helps the ground even more!
This the economic part of all this. There is no doubt in my mind that we have hurt our soil over the years. A tractor that could pull a 6 bottom plow 20 years ago, is lucky to pull a 4 bottom plow now. The soil is hard, tight, and dead. The life in the soil that makes it a good place for plants to grow is changed or gone. The earth worms that thrive in good soil have decreased. We need to do something to promote healthy soil. And if it is good for plants, it is good for us. I personally think that cancer rates have increased over the years due to little bits of lots of chemicals in almost all the food we eat, the air we breath, and the water we drink.
So, while we have become a nation or world of "limitless bounty", I think it is at the expense of something. I hope it is not at the expense of the Earth and life as we know it on the Earth. I know many farmers do not like to spray who knows what. They go out and enjoy working on the land they love and spray to help their crops and get chemicals on their hands and clothes, or drive their tractors and try to spray when there is a light wind and notice the mist of spray on their faces. That always scared me to death. I am happier not to have to use these things. Most farmers say or think "I cannot farm without chemicals. " Well, you are wrong. There are problems to overcome, but on the whole,farmers are among the most inventive and adaptable people on the planet. They have just been convinced over the years that they have to spend untold amounts of money each year for inputs that they really do not need. It's all about money. So if money is the problem, find a way not to give it to someone else. Start out with a few acres, and gradually increase til you get to the point where all your acres are chemical-free or where you can comfortably handle the management needed to farm the way our fathers and grandfathers did years ago. Also, there are many consumers out there who think it is important enough to eat healthy food that will be glad to pay a premium to get what you produce.