Theres is a difference between the dispatch fase and flight execution. In the dispatch fase you have to have enough fuel to arrive at the airport and have enough fuel to fly to the alternate and then hold for thirty minutes. In the actual flight fase it depends what you do. Sometimes diverting to the alternate may be better, sometimes burning up your alternate fuel is better, depends entirely on the conditions. It is definitely not required to divert right away!! You may use your alternate fuel at destination!!