I’m just a little guy but even so, compositing is probably the single most important technique that I use. Dunno, I probably over-use it. But I’ve never had nearly “enough” computer-power. From the very beginning I had to learn how to economize; how to do stuff without a “farm.”
I use the 3D software to produce the content, and I separate the various layers of possible output material: shadows, specularity, and so on. The idea is that once the computer has done this once, the rest of it is “tweaking.” Which is basically a two-dimensional mixdown process … ergo, “very fast.” There might be twenty or thirty channels of information going into a scene and there is a knob on every one. All of those channels of information were calculated ahead of time.
I do use Blender for most of that mixdown work. In other words, there is one file which produces the underlying layers, and another file which contains a compositing network, and believe it or not, Unix Makefiles (or equivalent Perl scripts, these days) which cause the appropriate re-renders and re-mixing to occur at the proper time.
So, “if all you want to do is to make that shadow just a little darker, or to change the color of that reflection,” you can do it, and you can do it now. Not quite right? Twist the knob a little bit and hit “Play” again. Instant gratification… I like that. 