This is a lesson for those who left their places, in which the Prophet (saws) put them, when they saw their companions were taking the booty, although they knew that the Prophet (saws) was there at the head of the believers, so definitely there would be no unfaithfulness in regards of the booty. So there is not any reason for the archers to leave their places, while they fully knew that their share of the spoils was preserved. Consequently, they caused the defeat in that battle, and more than seventy of the Companions were martyred. Mentioning the Day of Resurrection is followed with His (swt) saying:

ثُمَّ تُوَفَّى كُلُّ نَفْسٍ مَّا كَسَبَتْ وَهُمْ لاَ يُظْلَمُونَ {Then will every soul be [fully] compensated for what it earned, and they will not be wronged.}: So every person will be held accountable for what he has done:

وَأَن لَّيْسَ لِلْإِنسَانِ إِلَّا مَا سَعَى (39) وَأَنَّ سَعْيَهُ سَوْفَ يُرَى (38) ثُمَّ يُجْزَاهُ الْجَزَاء الْأَوْفَى

{And that there is not for man except that [good] for which he strives- % And that his effort is going to be seen % Then he will be recompensed for it with the fullest recompense} (An-Najm: 39-41)

Hence, every soul shall be compensated for what it earned, and the person shall not be wronged at all.

وَنَضَعُ الْمَوَازِينَ الْقِسْطَ لِيَوْمِ الْقِيَامَةِ فَلَا تُظْلَمُ نَفْسٌ شَيْئًا وَإِن كَانَ مِثْقَالَ حَبَّةٍ مِّنْ خَرْدَلٍ أَتَيْنَا بِهَا وَكَفَى بِنَا حَاسِبِينَ

{And We place the scales of justice for the Day of Resurrection, so no soul will be treated unjustly at all. And if there is [even] the weight of a mustard seed, we will bring it forth. And sufficient are We as accountant.} (Al-Anbiyaa: 47)