They are part of a more natural approach to growing food.
Microbes in the soil provide nutrients to your plants. If soil runs out of microbes then plants don't get as many nutrients as they could with the help of a healthy food web. Ensure the life in your soil has what it needs to thrive.
Some easy ways to get started:
Early planting of peas will be the first crop to brown off. Right after the first picking, plant oats. They will fill in fast and won't interfere when pulling the vines off their supports and lay them on the soil as residue. You can start a flat of spinach in late July and transplant them into the open spaces, just drop them right into the pea residues. Careful not to pull out the roots.
Greens don't take much from soil, so you can constantly plant more seeds throughout the summer. As we move into August, planting wheat will support soil fungi. Later in the month, planting daikon or tillage radish will further support the soil.
Harvest begins in July when scapes (slender white pods of bulbils) are ready and leaf tips start to brown off. If browning off occurs in July, it's a sign that your soil is poorly drained. Lift bulbs in poor health and plant oats. As you lift single bulbs, stick in a fava bean. When the rest of the crop is lifted, plant radish, Johnny Jump-up, or Stinkweed.
Spacing around bush beans is wide enough to try a variety of cover crops. You can stick in a few species of soil builders or food crops as soon as harvest begins in July. It's an opportunity to use up left-over seeds here and there and see if you get a later harvest (avoid big plants like squash). It's also an opportunity to build resources into your soil for next year. If you plan to grown corn in that spot next year, then plant Alfalfa and Buckwheat, or if you plant a root crop, then go with annual grasses like wheat and barley. The beauty of the combinations will surprise and delight you.
Don't forget to cover the bed with leaves in October.