While all things are related for a particular situation all things are not relevant. We need to develop a simplified model of reality which depicts those interactions which are responsible for the patterns of behavior and current situation. We want the simplest model which enables the requisite understanding. This is no easy task though. There is a also a video
for this insight which is a component of the Effective Problem Solving Course
We begin with a situation, or a problem if you will, and we really want to find a way to deal with it.
The difficulty is that the problem we need to deal with is typically embedded in a reality so complex as to be perceived as very complicated and often overwhelming.
We employ the situation to help us focus on just the relations relevant to the situation and we develop models. Models which are simplifications of reality.
The model is intended to promote understanding and when we get it just right the understanding surfaces insights which are far more profound than just understanding.
Because the model is a simplification of reality it must must be wrong in some dimension though the model needs to be sufficient to promote the requisite understanding.
You might consider this the essence of why developing models is so essential. Generally without models we end up lost in the maze of complex relations and everything just seems complicated.