Why is Rhizobium categorised as a 'symbiotic bacterium'? How does it act as a biofertiliser?

Rhizobium forms a symbiotic association with the root nodules of leguminous plants. The bacterium obtains food and shelter from the plant and the plant in turn, gets the fixed nitrogen. It multiplies as soon as it gets the rhizosphere of leguminous plants, enters inside their roots and forms root nodules. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as a nutrient.