But that's a command for the compiler and has nothing to do with SED.

I think if you load all your models and textures and so on via code written in SED the nexus is not used. It is only used if you load your models and textures beforehand in WED and create your levels inside WED. Then one big chunk of memory (the nexus) is allocated to hold all of your WED-made level data when you are playing the game, which speeds up loading and level switching.

This was useful back in the day (late 1990s, early 2000s).

I don't think the nexus is used for anything else but maybe some important stuff when the engine starts.