Turnout in a large paddock or field is one of the best things you can offer your horse. Horses have evolved as constant grazers and adapted to be on the move almost 24/7 with a herd. A lack of turnout has been linked to physical and mental problems in the horse, including ulcers, muscle and joint stiffness, and stress/boredom-related behaviors such as weaving and cribbing.
This video does a great job of explaining why turnout is crucial for horses, and how much turnout is the right amount for your horse:
