Why are root nodules useful for the plants?

Leguminous plants develop root nodules in response to infection by a specific genus of bacteria named Rhizobium. The nodules produce a molecule similar to hemoglobin in structure called leghaemoglobin, which is pink in colour. The leghaemoglobin acts as a carrier for nitrogen molecules and helps in it's fixation from the atmosphere via the roots in the plant. Thus root nodules are important for the plant to fix the atmospheric nitrogen.