Construction Script executes in editor, not at runtime.
It executes whenever level designer places actor in the level, or changes one of its properties.
It can be useful to help level designers with placement, visualization, auto-generating some component and/or values. Whatever you compute in CS is a gain for runtime.
Event graph doesn’t “run” per se. it’s just a container for events, and 99% of those events will be triggered at runtime only.
You may be able to call events from CS but that can be dangerous, as there are things you cannot do during CS (due to not being runtime), and only the CS is “aware” of it and will give you appropriate warnings about those.