Marketing reports and novels have specifications just as real as any physical engineering project, you're just required to guess at what exactly they are.

Whatever you're making, there's going to be some set of things that it must accomplish. It's much easier to make the correct thing if you know what those are in advance. The fact that acquiring specifications is hard, or that users don't actually know what they want doesn't change the fact that you want as precise a spec as you can get.

Often there are multiple options for what the thing you are making must accomplish (though some may be more valuable than others). I have found this fact liberating with regards to novel writing in particular, a case in which I personally would abhor a precise spec.

I do generally prefer very precise specs for engineering projects, of course.