They suppress weed growth and hold nutrients in the soil that might be washed away by Autumn and Winter rains. Some - particularly if from the bean family of plants - actually add fertility to the soil by fixing nitrogen - just like beans grown to eat. When they have done their work, start to flower or die back - they can either be dug into the soil to improve its structure, or provide a useful addition to the compost heap.
Finally - my personal view - I prefer to at least have something growing on my plot rather than empty beds. As long as it adds to the plot within broadly "green" principles - I'm keen on a Stakhanovite level of allotment plot productivity that would put a Stalinist collective farm commissar to shame ! ........
