Powdery mildew is a profit killer. When it hits in July that can mean $5,000 less in your bank account at the end of the season. Same crap with other diseases.
Fungicides are expensive. They take hours to apply and nobody enjoy spraying them. Worse, you need to reapply every 4 days. Often with little success. A better way is to go proactive. Don’t give them the conditions to grow in the first place. Keep the humidity low.
You invest in a greenhouse to extend the growing season. But, it's worthless if your tomatoes stop producing in September due to downy mildew. Allow them 2 more months of harvest with only 2 clicks.