See The Benefits of Growing Mustard from which the paragraphs below were extracted.
Numerous studies have shown that live mustard plant tissues, both seeds and roots, contain compounds that work as soil biofumigants by killing nematodes and pathogenic fungi. Reaping this benefit requires handling mustard like a green manure, because the beneficial compounds are released within hours after the plants are chopped down. But if you wait two weeks after turning under chopped mustard and then plant lettuce, you can expect a very productive crop with very few weeds.
Potato and vegetable farmers have begun using special mustard varieties as part of their rotation practices to suppress weeds and diseases. The method involves planting selected strains of mustard bred to produce high levels of glucosinolates in spring, and quickly chopping them up and turning them under in summer, when they reach full bloom. Used this way, mustard has a cleansing effect on soils that are carrying heavy pathogen loads. Mustard varieties to try for this purpose include Caliente, IdaGold and Kodiak.
For advantages and disadvantages see https://plantcovercrops.com/mustard-as-a-cover-crop/
Visit Mighty Mustard for additional information on mustard as a cover crop.