First, a "weed" is any plant that's growing where you don't want it. Sometimes they're lovely plants, even ones you'd pay money for in a nursery, such as forget-me-nots, but they've taken over a space reserved for something else. Or you just don't like them.
But weeds are often unattractive plants no one really wants to grow, like buttercups, various grasses, sheep-sorrel and others. Still even these have some good points.
First, they cover the ground with a layer of leaves that protects the soil. Bare soil gets hot, dry, or crusty, loses its microorganisms and its crumbly structure.
Second, they're often medicinal herbs. Dandelion, cleavers, plantain, chickweed and others have been used for both food and medicine. Even quack grass, the bane of my life, has its uses.
Third, weeds without perennial roots can be turned under as a "green manure" to enrich the soil. Do this a few weeks before planting to give them time to rot.
A bit like easy composting!
Watercress, an edible weed in a small ornamental stream