Why do we ask Allah for the good that is decreed for us, when it has already been written in the Preserved Tablet, whether we know it or not?

Scholars answer the question: "What is the benefit of supplication (du'a) when destiny (qada') is already written?" by saying that supplication is a cause for repelling بلاء (tribulation/affliction) and for the presence of mercy, just as sowing seeds is a cause for plants to emerge from the earth, and a shield repels an arrow. Thus, supplication is part of the destiny that repels tribulation.