<b>The Destruction of the Towns is a Proof of the Establishment of the Hour (Judgement)</b> 

Allah, the Exalted, is saying that in the destruction of the disbelievers and the salvation of the believers by us is,

إِنَّ فِي ذَلِكَ لايَةً

Indeed in that (there) is a sure lesson.

This means an admonition and lesson concerning the truthfulness of that which We are promised in the Hereafter.
 
لِّمَنْ خَافَ عَذَابَ الاخِرَةِ

for those who fear the torment of the Hereafter.

إِنَّا لَنَنصُرُ رُسُلَنَا وَالَّذِينَ ءَامَنُواْ فِى الْحَيَوةِ الدُّنْيَا وَيَوْمَ يَقُومُ الاٌّشْهَـدُ

Verily, We will indeed make victorious Our Messengers and those who believe in this world's life and on the Day when the witnesses will stand forth. (40:51)

Allah, the Exalted, also says,

فَأَوْحَى إِلَيْهِمْ رَبُّهُمْ لَنُهْلِكَنَّ الظَّـلِمِينَ

So their Lord revealed to them:"Truly, We shall destroy the wrongdoers. (14:13)

Concerning Allah's statement,
 
ذَلِكَ يَوْمٌ مَّجْمُوعٌ لَّهُ النَّاسُ

That is a Day whereon mankind will be gathered together,

This means the first of them and the last of them.

This is similar to Allah's statement,

وَحَشَرْنَـهُمْ فَلَمْ نُغَادِرْ مِنْهُمْ أَحَداً

And We shall gather them all together so as to leave not one of them behind. (18:47)
 
وَذَلِكَ يَوْمٌ مَّشْهُودٌ
 
 

and that is a Day when all will be present.

This means a day that is great. The angels will be present, the Messengers will gather and all of the creation will be gathered with their families. The humans, Jinns, birds, wild beasts and domesticated riding animals will all be gathered. Then the Most Just, Who does not wrong anyone even an atom's weight, will judge between them and He will increase their good deeds in reward.

Concerning the statement,

وَمَا نُوَخِّرُهُ إِلاَّ لاَِجَلٍ مَّعْدُودٍ