They can attract beneficial insects if they have attractive flowers such as dandelions.
They can improve the minerals in the nitrogen fixers, clover.
They can be edible - apparently chickweed is a good salad leaf as is dandelion. Brambles of course provide fruit but need to be kept in check. Nettles are also very nutritious and can be used in a potato champ or soup. In fact this was the first official crop from my plot - I made a delicious nettle and potato soup (basically chicken broth, cubed potatoes, onions, nettles and salt boiled up and chucked into the blender).
They can also create great fertilisers. Both comfrey and nettles can be turned into 'tea'. Pop the leaves into a bucket with water and wait for them to rot down a bit. Pongy, but a very effective fertiliser if diluted and fed to tomatoes, beans, sweetcorn and squash and other hungry vegetables.
I still haven't found a use for other weeds such as bindweed or couch grass but watch this space.
