To recenter the axle after a lift..
When you lift it changes the dynamics of the vehicle.. Since the track bar is mounted to the frame on one side when you lift it will pull the axle to that side. An adjustable track bar will push it back in place.. And it is good for your next lift to if you are lucky..
That is why you buy adjustable instead of fixed length . Because you can use the adjustable with the next lift where a fixed length would again be worthless when you go higher..
Hope that helps..
Just another soccer moms Grocery getter....