I'm still roaringly tumescent off the latest Football Manager podcast. I strongly recommend anyone interested in the DoF (both in-game and in general) check this podcast out. Joining Miles and the other dude from SI are a journalist from Norway, Lars Siversten (who manages not to speak like a Northerner,
Kalyl
), and the devil himself, Damien Comolli. It's brilliant.
TL;DR/L: The DoF is a response to the idea of a club as a "project", not just something that moves from year to year with no overarching plan or idea. Comolli also points out that every club that won a medal last year in England (or won promotion to the PL) uses a DoF. The title might be different, and the responsibilities certainly vary, but there's someone in place who is less volatile than the manager (who averages 18 mos in his post).