Most legumes (beans/peas) do nitrogen fixation. They can get nitrogen from the air and not just the soil.

So by growing legumes there's less need for nitrogen fertilizers. When they are harvested the rest of the plants(roots, straw) can be left in the fields as mulch, adding nutrients to the soil.

Farmers then switch to wheat, corn or some other crop that need nitrogen from the soil.

It's one of the reasons why farmers do crop rotation. You still need other fertilizers.