Because Writing is Thinking, mindmapping is an excellent tool for initial exploration of subjects.
Austin Kleon described this further: As it feels closer to drawing, it is much lower-barrier to put thoughts into writing
It serves as a form of “free writing” — it gets things out of my head quickly so I can look at them on the page. (I think because you’re starting in the middle and working out, the radial pattern tricks your brain into loosening up.)