I was taught that, for a farmer, weeds are Nature's support crops and are vital to a healthy system. Now, there is a point in the beginning of our cash crop's life where we have to fight for its little life and take down the weeds, but once the cash crop is big enough to not be overpowered by the weeds, let them grow.
Weeds come in all shapes and sizes, both on top and root structure. Some weeds are vines to cover and protect the soil, some are tall and woody, some have thin and abundant roots, like grasses, and some have a single tap root, like a turnip. Each weed has its sole purpose, which is to grow, flower, and produce babies. The structural makeup of the plant will tell you the purpose. Like grasses are designed to hold soil in place with their many fine root hairs, and a tap rooted plant is designed to break up compacted soil. The thing that separates weeds from our garden plants is they grow much faster. They grow faster by design; their main purpose is to build soil. Some species of weeds, called pioneer species, not only grow fast, but they produce carbon quickly as well. This carbon, when they die, lasts a long time in the soil, helps build structure, and helps retain water. Weeds have certain nutrients that they absorb from the soil, bring to the top, and release when they die or compost.
So looking across an abandoned field you will see a variety of weeds growing, each of them is a reaction of whatever deficiency the soil has. Over time the field will eventually become a jungle, or whatever the geographic location dictates. For example, the taller weed that has a small daisy type flower growing in our area is known for drawing calcium out of the soil. So an abundance of this plant is an indicator that your soil is lacking calcium. I have encountered farmers that can look at a field and tell you what your soil needs just by looking at the weeds.
Other big reasons for weeds in the garden
Where in nature do you see bare soil, besides the desert? Weeds protect the soil and help build it, but there is another main reason to keep them around. A plant's main purpose in life is to produce sugars; they do this through the process photosynthesis. Remember that word from grade school? Well the sugars they produce go to making leaves, flowers, and eventually fruit and seeds, but as much as 50% of the sugars they produce gets exuded though the roots to attract the microscopic organisms of the soil food web. Plants do not have a stomach, so they rely on the soil to digest and transfer nutrients. Plants also, do not have a stomach liner to keep the digestive process in the root zone, so they exude these sugars to attract the biology in and keep them there to feed the plant.
Weeds also help protect surrounding plants. Sometimes weeds will help hold up our plants, or maybe they will shade them from sun, or maybe they will take the insect attack for us. In a lettuce field weeds are super important for another reason. They keep the soil out of the leaves. If the weeds were not present, when we irrigate or a rain comes the water droplets splash the soil in the leaves. When we have weeds surrounding the lettuce, the soil is not bare and therefore the weeds help keep the dirt from splashing into the leaves. Believe me, growing lettuce without weeds is a big problem. Delicate leaves are hard to rinse thoroughly and no matter what your lettuce will be crunchy from the sand and dirt.
Plants work together in their little community and biodiversity is the key. Bare soil is rare in nature and so is a monoculture, diversity is key. Plants work together and help feed each other, if you learn how they grow and how to cultivate them you can benefit from Nature's support crops in your garden.