[zozbot234]

Nitpick, but the bass part does not always play the root of a chord in the proper sense - "inversions" (which are actually very common) are precisely those chords where the bass is playing some note other than the root. What is true however is that the bass part provides the broadest level of "structure" in a piece of music that can be directly read in the score. Notes in other parts are always (at some basic structural level, disregarding further elaboration) chosen based on how they "relate" to the bass according to the rules of counterpoint.

The "thoroughbass" is a historic way of thinking about the "accompaniment" of a piece of music that's based on these principles; a "thoroughbass part" writes down chords by "annotating" a bass line with intervals, and music pieces can be written as "lead line(s) plus thoroughbass part" in a way that's comparable to a modern lead sheet but a lot more theoretically founded.