Ideally, a song should contain both elements of high melodic tension, and low melodic tension. No listener wants to sit through a totally high-energy 180 BPM non-stop 6-minute ride through synth mania unless they are already busy grooving madly on some dance floor in a smoky club somewhere. Also, unless your listener is on heavy sedation, he or she will not enjoy your sparse 18-minute ambient tune which consists of the same languid piano riff repeated over and over again.