Game Theory: When to use primary execution in the controller controller

When should you use the “Mark Controller for execution before all unmarked” Button?
I know when, because I need to do so for my scope rendertotexture to work once the menu scene switches to my game scene.
I just want to know when to spot another one of these.

I have a game that works well, but if I add it to a menu scene as another scene and have the menu scene switch over to game scene, then some things don’t work.

How do I spot when to use it?

You can use it if you want to guaranty something to set up, that other (non-priority) controllers rely on. Usually you should try to avoid such dependencies, but this is not always possible.

In the past I used it to set up the Python search path. That had to run first. Otherwise the Python controller did not find the Python files. Luckily I found a much better solution, so it is not necessary anymore.

I also used it as development help. E.g. to print the frame number right before any other output of the same frame.
But I found better solutions to this aspect too (logging + a property).

Btw. As far as I remember this works within a single scene only as the scenes are executed on after the other.