I don't know why exactly it's helpful, but it definitely is.

When you keep it in your head, you don't have to form words; you can just think about what's bothering you as a vague concept. When you verbalize it externally, it forces you to clarify those ideas and pinpoint exactly what you're thinking; as far as I can tell, that's where the utility of the technique comes from.

I recently rediscovered this as a means, not of solving technical problems, but of overcoming strong negative emotions. Even when I already understood the facts and causes, "talking it out" on the page helped me vent the stress and calm down.

Perhaps in situations where your emotions are inhibiting your thinking, writing the useful parts (what you think, want, and can do) but not the useless parts ("oh god oh god everything is terrible") gives the former more weight.