The 2 virtues of saying the following du`aa

بـسـم الله والحـمـد لله والـصلاة والـسـلام عــلى رسـول الله، وبـعـد

{رَبَّنَا آمَنَّا فَاغْفِرْ لَنَا وَارْحَمْنَا وَأَنتَ خَيْرُ الرَّاحِمِينَ}

{Our Lord! We believe so forgive us and have mercy on us, for You are the Best of all who show mercy} [Surah al-Mu’minoon (23): 109]

1. First: It is the command of Allaah تعالى to say it:

{وَقُل رَّبِّ اغْفِرْ وَارْحَمْ وَأَنتَ خَيْرُ الرَّاحِمِينَ}

{And SAY (O Muhammad صلى الله عليه وسلم): “My Lord! Forgive and have mercy, for You are the Best of those who show mercy!”} [Surah al-Mu’minoon (23): 118]

2. Second: Allaah تعالى will mention them with honor: The denizens of Hell will ask Allaah تعالى to take them out of it, but Allaah تعالى will address them saying: You were mocking at them and forgot My remembrance, while they were seeking My forgiveness:

{وَمَنْ خَفَّتْ مَوَازِينُهُ فَأُولَـٰئِكَ الَّذِينَ خَسِرُوا أَنفُسَهُمْ فِي جَهَنَّمَ خَالِدُونَ * تَلْفَحُ وُجُوهَهُمُ النَّارُ وَهُمْ فِيهَا كَالِحُونَ * أَلَمْ تَكُنْ آيَاتِي تُتْلَىٰ عَلَيْكُمْ فَكُنتُم بِهَا تُكَذِّبُونَ * قَالُوا رَبَّنَا غَلَبَتْ عَلَيْنَا شِقْوَتُنَا وَكُنَّا قَوْمًا ضَالِّينَ * رَبَّنَا أَخْرِجْنَا مِنْهَا فَإِنْ عُدْنَا فَإِنَّا ظَالِمُونَ * قَالَ اخْسَئُوا فِيهَا وَلَا تُكَلِّمُونِ * إِنَّهُ كَانَ فَرِيقٌ مِّنْ عِبَادِي يَقُولُونَ رَبَّنَا آمَنَّا فَاغْفِرْ لَنَا وَارْحَمْنَا وَأَنتَ خَيْرُ الرَّاحِمِينَ * فَاتَّخَذْتُمُوهُمْ سِخْرِيًّا حَتَّىٰ أَنسَوْكُمْ ذِكْرِي وَكُنتُم مِّنْهُمْ تَضْحَكُونَ * إِنِّي جَزَيْتُهُمُ الْيَوْمَ بِمَا صَبَرُوا أَنَّهُمْ هُمُ الْفَائِزُونَ} 

{And those whose Scales (of good deeds) are light, they are those who lose their own selves, in Hell will they abide. * The Fire will burn their faces, and therein they will grin with displaced lips (disfigured). * “Were not My Verses (this Qur’aan) recited to you, and then you used to deny them?” * They will say: “Our Lord! Our wretchedness overcame us, and we were (an) erring people.” * “Our Lord! Bring us out of this. If ever we return (to evil), then indeed we shall be Zaalimoon (polytheists, oppressors, unjust, and wrong doers).” * He (Allaah) will say: “Remain you in it with ignominy! And speak you not to Me!” * Verily, there was a party of My slaves who used to say: “Our Lord! We believe so forgive us and have mercy on us, for You are the Best of all who show mercy!” * But you took them for a laughing stock, so much so that they made you forget My remembrance while you used to laugh at them! * Verily, I have rewarded them this Day for their patience, they are indeed the ones that are successful.} [Surah al-Mu’minoon (23): 103-111]