When rules are implicit, only those who know how decisions are made can participate. The only way one can know how decisions are made are to spend enough time cultivating friendships with the elites that establish the informal structures of the group. Therefore, informal structures are exclusionary (one must be a member of the elite) and have other negative consequences.
Formal structures counteract these issues by making decision making explicit. Everyone now has the opportunity to participate because it’s more clear how to do that.