What is the wisdom behind mentioning the terrors of the Day of Judgment? What is the meaning of a mirage?

The wisdom behind mentioning the terrors of the Day of Judgment is to curb the desires of the disobedient and to instill fear in them so that they may follow the straight path, and to establish the proof against them. As for the obedient, bliss is mentioned to them so that they may increase in their eagerness for what pleases Allah, and the disobedient are reminded of the torment so that they may repent and return. A mirage is what appears to the beholder but has no reality, as in His Almighty saying: "Like a mirage in a lowland which the thirsty one deems to be water until, when he comes to it, he finds it to be nothing."