I think that the stuff in our mind is mostly abstract. We absorb things from the world all the time, conscious and unconscious, and make conscious and unconscious connections between them.

Thinking aloud, whether in the mind, on paper or even vocally, helps us to identify some of these points, and thus quantify a meaning from it. There's only so many points that we can mentally hold though - we're not good multi-taskers after all! So the better we can think aloud, the better we can consciously grab thoughts and identify the real gaps - which we can fill in by searching/ creating more points in our mind, or go external.

Hmm, just thinking aloud though ;)