Every farmer worth his compost knows that a sabbath rest for the land is not just a good idea, but a basic requirement of agriculture. The cycle of sowing, cultivating and harvesting takes its toll on the soil, microorganisms and variopus insects and other animals that live there.
Giving the land a rest once every seven years helps the microecosystem to restore itself, for the soil to regain lost nutrients and for your next crop to be even better. It's even more important if you rely on chemical fertilizers and pesticides, which "burn" the soil and damage beneficial organisms in the ground like earthworms and ants.
It's a practice I'm trying to apply with my organic garden and flower beds at this house, though I'm starting with soil that's been badly abused over the 80-plus years this house has been in existence. I left alone for two years a nice swath along side of the house and in the back yard where nothing has been growing, and last year, I started to see a few things start to grow there on their own. They'll be even further ahead when I mix in compost this spring.
Of course, I'm not aware of anyone who gives ALL the land such a rest at the same time, which is what the Torah prescribes. Usually it's cycled through.