The first cousin to Cultivators, Plows are used to both prepare the ground for sowing AND break new ground to prepare it for use as a field for goring crops.
You only actually need a plow if you want to break new ground. You may be tempted to do that - enlarge a field - by cutting the trees and plowing it up but for the most part that does not work very well due to the roads. If the field could have been made larger they would have.
That said though, you CAN use the plow to connect two fields into ONE which may be a very good idea. Just saying...