The line tension is what it is, but it's affected some by the angle the airplane is yawed and it affects that angle too. Adjustable guides can be moved to that angle. More to the point, fixed guides can pull the model toward an inefficient angle. With adjustable guides, you can easily trim the yaw, or at least the affect of the yaw.
In the really old days, guys very often cut the fixed leadouts and moved them when the plane flew crappy because they'd guessed wrong and placed the leadouts badly. Sort of labor-intensive adjustables
