The above answer is even more true if you have consumed a very acidic drink, eg fruit juices, carbonated drinks. The acid present in the drink will weaken the tooth enamel. Brushing straight away will cause more damage to the weakned enamel. By waiting at least 30 minutes, it allows saliva to raised the pH and neutralise the acid, and to remineralise the enamel to make it more resistant ot brushing.