Apart from the obligatory five daily Prayers, Muslims have been exhorted to offer voluntary Prayers in order to supplement the shortfalls that usually occur during obligatory Prayers. Also, performing nafl (supererogatory) Prayers increases the Muslim’s reward and draws him nearer to his Lord.
As for observing obligatory prayer with the intention of nafl prayer, some Muslim scholars allow a person to pray an obligatory Prayer for a second time while intending it to be a nafl.
Shedding more light on this, Sheikh Ahmad Kutty, said:
This is a contentious issue among scholars. However, we have authentic traditions from the Prophet (peace and blessings be upon him) that he allowed Mu`adh to lead salah after he had prayed behind him in his mosque; so, for him, it was a nafl Prayer, but for those praying behind him, it was a fard.
Based on this precedent, many scholars are of the view that this can be done. Where there is an authentic hadith, we need not adhere rigidly to fiqh rulings that contradict it.
There is also precedent from the Prophet (peace and blessings be upon him) for someone praying in congregation after he had prayed at home; so while it was fard for the imam, it was nafl for the one who prayed the same salah behind him for the second time.