Subscenes and their use

I recently switched back to using Unity and am trying to wrap my head around the concept of subscenes (and ECS) but aside from some forum posts here and there and the official documentation I haven't really found a lot of information about it.

How should subscenes be used (as in, what kind of scenarios should I be thinking of)? Do I just have one main scene and a bunch of subscenes that I switch between? Or are they basically modules that I can load/unload that, together, form the actual scene (is that what open/close/unload is for? To make them only load or be active when they're actually needed?

Thanks!