Seeking forgiveness is demanded in all the times because the man is not free of committing sins in any time, even failure in worshipping is considered a sin.

This command is directed to the example, and the teacher, the Prophet (saws), so that every nation will do it after him because in the verse which will come afterword Allah (swt) says:

  وَلَوْلَا فَضْلُ اللَّهِ عَلَيْكَ وَرَحْمَتُهُ لَهَمَّت طَّائِفَةٌ مِّنْهُمْ أَن يُضِلُّوكَ

(And if it was not for the favor of Allah upon you, [O Muhammad], and His mercy, a group of them would have determined to mislead you.) (An-Nisa: 113)

Because some of the people has not accepted the judgement of the Prophet (saws) when he accused the Muslim man and acquitted the Jew and said this reflect a bad reputation, so that Allah (swt) asks the Prophet to tell those who claim that or think of it to seek forgiveness of Allah (swt) because this is not acceptable matter.

( إِنَّ اللَّهَ كَانَ غَفُورًا رَّحِيمًا … ) (…Allah is ever Forgiving and Merciful): Allah (swt) has been and will ever be Forgiving and Merciful because no changes whatsoever occurs to Him (swt), the Lord of mankind.