Legumes are different from grasses because they form a symbiotic relationship with bacteria called rhizobia in the nodules of their roots. This symbiotic relationship allows legumes to fix atmospheric nitrogen into a form that is available to the plant, which in turn makes them an excellent source of nitrogen for other plants, including grasses.
Grasses, on the other hand, do not have this ability to fix nitrogen and rely on other sources of nitrogen such as fertilizers or nitrogen from the atmosphere that has been fixed by other means.
In addition to their ability to fix nitrogen, legumes are also an important source of protein and other nutrients for both humans and animals. This is due to the fact that they have a higher protein content than most grasses.
Therefore, legumes and grasses serve different purposes in agriculture and can be used together in a complementary way to improve soil fertility and provide a balanced source of nutrition for livestock and humans.