There's no commercial reason: it doesn't carry ads and it doesn't promote the advertising agency. But if you turn to page 9, the agency's strategy director, Dylan Williams, gives it away: "We've got a real buzz out of making it."
In the same vein, why do BBH and Abbott Mead Vickers BBDO let their creative staff devote so much of their working day to their personal blogs, Scampblog and Lunar BBDO respectively? The reason is simple: great agencies thrive on great creatives. If their appetite to create goes beyond their day job, the best shops indulge that urge. In addition, the blogs all serve to present their respective employers' creative departments as great places to work.