So I think I can understand that maybe you make a major change in a project to one of the assets used in the renders but is that the only reason why you'd have the compositor option on in the post processing options and then re-enter with blender using your nodes to adjust what's in 3d?
Basically in compositing you can do a lot of post-render operations. It has it's limitation, but the whole VFX industry depends on it, it is its lifeblood. When you see all those Marvel behind the scenes and everything is green screen, it's through the magic of compositing that all the VFX gets added. So yes, to adjust what's in 3D and much much more.Â
In my very limited beginner experience the compositor is useful to know and be able to use even if you don’t need colour correction and/or green screen.Â
You’ll realise it’s usefulness when you need to do something in a project - your desire to have only bloom applied to selected emission materials is a case in point.Â
In my case it was to get shadows from the plane/ground you add to you viewport but then switch out this background and keep the shadows.Â
Recently I had another problem where I wanted to mask out an element of an object from my render which I could do with a holdout shader but that also masked the background so I needed to composite the masked render back to the background layer. Being able to do this within the render is so much more efficient than rendering out 2 files and manually putting them into photoshop cos you’d have to do this every time you re renderedÂ