In general, they don't contain as much protein or fats(2), but they bring in high doses of varied other nutrients(1). Fiber, big ranges of vitamins(2,3) and minerals, and healthy carbs(2) are hallmarks. They are vital defenses versus unhealthy aging and multiple diseases(3). Another important set of nutrients, antioxidants, are cardiovascular health necessities - the very linings of our lungs and respiratory tracts alike need a steady range of them. Fruits, veggies, and the antioxidants they contain therefore are believed to help alleviate symptoms of asthma and allergies(4). Proper storage and handling, of course, are important to prevent some issues that may occur otherwise(2) - and may be excuses for why some don't eat proper amounts.
So, to wrap up, besides fiber and other healthy carbs, fruits and veggies are simply little worlds of vastly varied nutrients that fit more on the bottom portion of a Nutrition Facts label than the top - but very crucially add up.
Sources:
1. "Fruits & Greens Focus: Are You Getting 'Variety'?" Isagenix Health, Isagenix International, LLC, 6 March 2012, https://www.isagenixhealth.net/fruit-greens-focus-are-you-getting-variety/
2. "Vegetable." Wikipedia, Wikimedia Foundation, Inc., https://en.wikipedia.org/wiki/Vegetable
3. "Fruit." Wikipedia, Wikimedia Foundation, Inc., https://en.wikipedia.org/wiki/Fruit
4. Greger, Michael. "How Fruits And Vegetables Can Treat Asthma." VegSource, VegSource Interactive, Inc., 2 July 2015, https://vegsource.com/michael-greger-md/how-fruits-and-vegetables-can-treat-asthma.html