Watch the Partial Capo online guitar lesson by Stephen Bennett from Fingerstyle Finesse

This segment talks a bit about the idea of the solo acoustic guitar fulfilling the multiple functions of a rock and roll band - driving the rhythmic component of the music (by implying a bass player and drummer), as well as filling in the harmonic component (implying a rhythm guitar or maybe keyboard), as well as the melodic component (a lead guitar or vocal).
It's an imperfect sort of comparison, but it is just one guitar playing and if you listen, you CAN hear these various pieces that make up the whole.