Chris Harman, Revolution in the 21st Century, London: Bookmarks, 2007. Pages 73-74:
[A]ny genuine mass movement involves a wide array of people with differing views on what needs to be done. Many argue for the line of action they think correct. Someone suggests a demonstration or strike. Someone else thinks such action is premature and there should be further negotiations. A third person wants no action at all. A movement may appear spontaneous to an outside observer but viewed from the inside it invariably involves attempts by myriad individuals to lead in different directions.
Parties would come to exist even if the divergences between people were simply random, but they never are. The divergences are structured by the pressure of existing society on the protests that arise - above all by the assumptions engrained in people’s consciousness that things cannot be fundamentally different to the past. Conservative currents argue society cannot be changed at all and reformist currents that it can only be changed in part. These currents form spontaneously through the impact of existing society, and are encouraged by the media and by those with careers tied to the existing political institutions. So when people say there is no need for parties, they are saying there is no need for revolutionaries to get together to oppose these currents…