What's the point of documentation?

It is a truth universally acknowledged, that software documentation [Lethbridge et al, 2003] is a Good Thing, and spreadsheets are no exception [Morison & Jordan, 2000]. The FSA, in a recent newsletter [FSA, 2006], described what they had seen in the way of good practice for financial modelling systems: “Acceptable standards of documentation were established, agreed by the firm, and themselves documented.” They went on to say “The standards of control and documentation applicable to systems developments applied equally to spreadsheets”.