Mustard plants are used as a cover crop in vineyards to reduce erosion, add nitrogen and improve soil tilth and water penetration. Cover crops helps to increases organic matter, reduces compaction, reduces excessively vigorous vines, dust and mite problems, & provides a habitat for beneficials. By providing firm footing during wet weather, the plants allow for winter/early spring access in the vineyard.
Mustard is one of several "cover crops" growing between
rows of grapevines in the Napa Valley, and Sonoma Valley, California because:
"Mustard helps to control something called 'nematodes,' which are small worms in the soil that will try to eat the plants."
Later in the Spring, they will mow the Mustard, this is called "Cuttin' the Mustard." :-)

"Studies show that several different Brassicas including Indian mustard (B. juncea) have been tested as green manures for controlling root-knot nematode in vineyards. According to research in Australia, Brassicas such as mustard, cultivated as an inter-row green manuring cover crop contribute to the suppression of nematodes."