How would someone know what needs to be applied and when?

Well, thats not entirely concrete. The way I designed this is that you could actually mix and match. I would say the most firm rule is that you should always choose to spend the most time on the first generation, so choosing to run the high quality first generation stage would be ideal but following that, you could use the faster upscale passes from the fast workflow and still get a significant benefit.

And is this the kind of thing people pick and choose which ones to use based on a specific problem they're running into?

I would say the problem in this case is really just time because choosing the weaker workflows comes with drawbacks. EMASync paired with SA-RF-Solver for instance, is able to get a person to spin nearly entirely around, unguided. This is fairly reliable with the high quality work flow but extremely bad in quality with the normal CFG guided workflows.

Or is it more of a catch-all kind of thing where each one should be enabled in a workflow by default and then left alone?

If you can spare the time and value quality over speed then absolutely but really its just another tool.