Why do farmers plant leguminous crops in crop rotations/mixed cropping?

After continuous plantation of the crops like rice and wheat, the nitrogen level of the soil decreases. This, in turn, affects the yield of the crops. The leguminous plants have nodes in their roots where the nitrogen-fixing microbes reside. These microbes help by fixing the nitrogen in the soil. Hence increasing the nitrogen level of soils. So the farmers plant the leguminous plants to maintain the nitrogen level of the soil.