This paper proposes an answer to the title question on the basis of the analysis of empirical data -- a large corpus of what I call thought sequences, namely, trains of verbal-like expressions that spontaneously pass through people's minds. The analysis reveals several patterns that could not have occurred had thought not been conducted in a conscious manner. The feature that makes these patterns possible is the concreteness resulting from the articulation of thought in a particular medium: such articulation is perforce conscious. In practically all standard models of cognition today the substrate of cognitive activity is abstract, and, indeed, consciousness is usually not accounted for (in fact, cannot be accounted for). Here, I show that non-abstractness of mentation provides for three important functional benefits. First, the local de-coupling of medium and content opens the possibility of thought progressing along lines not planned or envisioned by the thinker beforehand, and thus it is a key for the generation of novelty. Second, articulated thought creates a medium for activities carried out in the internal theatre of the mind that are analogous to activities carried out in the real world. Third, articulation provides for the quality of entitihood, hence for compartmentalization and enhanced control as well as for reflection and meta-observation. The discussion is grounded in a general critique of the conceptual foundations of cognition that regards action in the world, not computational operation applied upon abstract underlying symbolic representations, as the basic capability of the human cognitive system.