It takes a village to make a videogame, and there's no one way to do it. Solo devs and quintuple-A studios alike face all sorts of challenges trying to make all a game's moving parts cohere, and the more cooks you add to the kitchen, the more different design philosophies are bound to conflict. That's where design documents come in—living, comprehensive treatises that outline all a game's systems and design goals to be referenced by those working on the game—at least, for some.
Johan Andersson, lead on Europa Universalis 5, said in an interview with PC Gamer news writer Joshua Wolens that the practice can be too rigid and restraining. For Paradox Tinto, good design comes with a bit of democracy.
