Straitnickel,
CL models can be thought of a having all their weight concentrated at a single point - the center of Mass, usually called the CG (Center of Gravity.).
Balance points can be found by hanging or supporting the model where it lines up straight below (or above) a point in three "axes." Relax, this should stay simple - because it is.
We all know "where the CG" (center of gravity) should be; there's usually a symbol that looks like a BMW logo, or wording to "balance the model x inches back of the leading edge," or some such. That only considers where the "Mass" should balance when we look at the side view, and it is vitally important for controllability AND flyability. The line pull will always tend, or try, to line up to the CG. But consider...
If the model is a high-wing type (can you say Piper Cub?) and your leadouts come out through the wingtip, when you hang the model by the leadouts it will bank. The vertical CG is not in line with where the leadouts enter the model, so it will not hang vertical, but try to tilt to where the mass IS directly under the leadouts. If you fly a model with the vertical CG way off, it WILL bank to try to line up the line pull with the leadouts. For a high wing model, that is a bank IN, towards you at the center of the flight circle. That will try to turn it in towards the center, reducing the line pull you need to control flight! NOT good!
The third "axis" is yaw - nose left or right of where you want it to be. ...Or just where you want it to be so the model flies as nearly as possible with the long axis of the body "tangent" (remember high school geometry?) to the flight circle. Adjustable leadouts let you "dial in" their location so that the line pull doesn't cause a nose-in yaw - reducing pull too much. ...OR cause too much nose-out yaw, which causes drag (dragging the model around kinda sideways) and messes up stability as well as maneuvering. The 'ground test' for good leadout location is to assure that the model DOES hang slightly (SLIGHTLY!) nose-out when hung by the leadouts. That is, the fuselage centerline should hang 2° to 3° or 4° nose down when hung that way.
...the reason is simple, too. The pull tries to aim at the center of mass - CG - , from where it reaches the leadout guides. The lines sag behind a straight line from the handle, because they have drag from flying through the air. Lines - thin wires - can't push sideways, but they can AIM the pull on them along their length. We want it to go through the front-to-back CG location, right? If it aims too far forward, it will try to pull the nose into the center - not good, right? If it aims too far back, we're back to the condition of dragging the model around part sideways to the direction of flight.
These thoughts are important because line pull is the strongest force applied to the model in smooth flight (sharp maneuvers bring their own problems, but if you are "centered" between the extreme cases with those, at least the effects will increase similarly for "inside" and "outside" turns as you go further from smooth level flight.) Thrust? remember, only enough thrust is applied to the model to overcome the fairly small drag; in level flight, that's not a lot. Take-off acceleration and meeting maneuver drags are temporary loads, which drop back to the low drag smooth level flight condition.
So, summing up: adjustable leadouts can help you tweak your model to a better flying condition, which is always welcome. They are a slight bother to build in, but well worth the weight and bother to be sure it won't destroy your enjoyment, or worse: your model.