Why might farmers plant legumes such as peas to improve the nitrogen levels in their soil?

Because of the presence of root nodules.

Root nodules are said to house bacteria that are able to fix atmospheric nitrogen. Though nitrogen is abundant in nature, they are in a form of relatively inert gas which plants are not able to utilize. The bacteria living in the root nodules are then fixing the atmospheric nitrogen making them available to plants.

Application of inorganic fertilizer especially nitrogen fertilizer is the common practice of farmers to improve the yield of crops. However, the long-term oversupply of nitrogen fertilizers has resulted in many environmental risks and damages. Thus, farmers are planting leguminous plants such as peas to enhance nitrogen levels in soil and improve crop yield while not imposing risks of environmental pollution.