The track bar locates the body / frame in relation to the front axle side to side. The track bar is mounted from the passenger side of the front axle up to the driver side frame rail. When lifted, the front axle shifts toward the drivers side. This is definitely noticeable with lifts over 4 inches - 3 inches to 4 inches is considered borderline. If the vehicle is already lifted, view it from the front and see if the tires are centered underneath the body. Some people try to correct this by sectioning and lengthening the stock bar.
You want the angle of the steering drag link and the angle of the track bar to match, if they do not then you can have "bump steer" when driving the vehicle. If you have installed a drop pitman arm or it is commom to swap over to a 2wd Ford power steering box on these early box Bronco's. Both of these will lower the mounting point of the steering drag link and a track bar drop bracket will help to get the angles of the track bar and the steering drag link to match.
For example if you have installed a 2-3 inch lift kit and do not have a drop pitman arm or the 2wd ford power steering box conversion, the best way to get the front axle centered and keep the steering and track bar angles proper is to use the adjustable track bar. If you have installed a 4-5 inch suspension kit, you should have a drop pitman arm and then should install the track bar drop bracket. You can install the adjustable track bar with the drop bracket to give you the ablitiy to perfectly center the front axle and also have the angles for the steering drag link and track bar as close as possible.