ai – Behavior tree: why?

I am new to UE4 and game programming altogether. Following multiple tutorials and documentations I see mentions of behavior trees. In tutorials they do not use them in favor of blueprints because they do not want to complicate it.

I can find many resources about behavior trees with “HOW” but I can not find any “WHY”. So my question is, why does it exist and why should I learn it, or if I even should? What are advantages of it comparing to blueprints or even writing my own code?