Extended Growing Season
This is the single biggest reason to use a high tunnel. You can start plants in a high tunnel during the early part of the season when they would die or fail to sprout in unprotected ground. The high tunnel provides some solar gain, allowing the sun to warm the earth within it. You can extend the growing season into the fall and winter as well. This longer growing season means more income for your farm.
Protection From Weather and Pests
Although not quite like growing in a totally pristine environment, high tunnels offer some protection from weather, like high winds or heavy storms, as well as pests.
Even if you don't use it for larger crops, a high tunnel can be a great tool for starting seeds for the small farm on a bigger scale than you can achieve in your farmhouse on a seed shelf.
Growing Alternative Crops
The high tunnel can be a place to experiment with alternative crops that may require different soil or growing conditions than the rest of your farm. A high tunnel can be the perfect spot for these crops to be isolated and trialed.
Compared to a greenhouse, a high tunnel is cost-efficient. Because they don't require a heating system, you can save on the cost of figuring out heat and installing it.
More Spacious Than Cold Frames and Polytunnels
For those who don't have anything else, cold frames and polytunnels can provide some season extension. But they can be difficult to maneuver under and require more attention and fussing. You can't walk into a cold frame. You can walk into a high tunnel, providing much greater access plus a larger area for growing crops.
Because a high tunnel shields the ground below from rains, you will need to provide irrigation via an automatic irrigation system. And although this is an expense, it gives you a chance to control how much water your crops get. In some cases, this can be very handy.