The guitar offers the ability of a solo performer to play highly complex, multi part music, perhaps second to only the piano, in terms of its contrapuntal ability (ie the number of independent parts it can play).  The guitar, like most stringed instruments allow the player to vary the timbre as part of its expressive power. For example by plucking the strings closer to the bridge produces a tighter brighter tone ,  while playing closer to the fret board give a warmer tone. Using these tonal variations allow the guitarist to effectively orchestrate music by contrasting and highlighting different passages.