1- It is a cause to receive the Love of Allah azza wa jal. The Most High says:

إِنَّ اللَّهَ يُحِبُّ التَّوَّابِينَ وَيُحِبُّ الْمُتَطَهِّرِينَ

Truly, Allah loves those who turn unto Him in repentance and loves those who purify themselves. (2:222)

2- It is a cause of success. The Most High says:

وَتُوبُوا إِلَى اللَّهِ جَمِيعًا أَيُّهَ الْمُؤْمِنُونَ لَعَلَّكُمْ تُفْلِحُونَ

And all of you beg Allah to forgive you, O believers, that you may be successful. (24:31)

3- It is a cause of the acceptance of the deeds of a slave and pardoning of his sins. The Most High says:

وَهُوَ الَّذِي يَقْبَلُ التَّوْبَةَ عَنْ عِبَادِهِ وَيَعْفُو عَنِ السَّيِّئَاتِ

And He it is Who accepts repentance from His slaves, and pardons sins. (42:25)

and The Most High says:

وَمَن تَابَ وَعَمِلَ صَالِحًا فَإِنَّهُ يَتُوبُ إِلَى اللَّهِ مَتَابًا

And whosoever repents and does righteous good deeds; then verily, he repents towards Allah with true repentance. (25:71)

4- It is a cause of entrance into Jannah and safety from the Hell-Fire. The Most High says:

فَخَلَفَ مِن بَعْدِهِمْ خَلْفٌ أَضَاعُوا الصَّلَاةَ وَاتَّبَعُوا الشَّهَوَاتِ ۖ فَسَوْفَ يَلْقَوْنَ غَيًّا

إِلَّا مَن تَابَ وَآمَنَ وَعَمِلَ صَالِحًا فَأُولَـٰئِكَ يَدْخُلُونَ الْجَنَّةَ وَلَا يُظْلَمُونَ شَيْئًا

But there came after them successors who neglected prayer and pursued desires; so they are going to meet evil. Except those who repent, believe and do righteousness; for those will enter Paradise and will not be wronged at all. (19:59-60)

5- It is a cause of forgiveness and mercy. The Most High says:

وَالَّذِينَ عَمِلُوا السَّيِّئَاتِ ثُمَّ تَابُوا مِن بَعْدِهَا وَآمَنُوا إِنَّ رَبَّكَ مِن بَعْدِهَا لَغَفُورٌ رَّحِيمٌ

But those who committed evil deeds and then repented afterward and believed, verily, your Lord after (all) that is indeed Oft-Forgiving, Most Merciful. (7:153)

6- It is a cause in the changing of evil deeds into good deeds. The Most High says:

وَمَن يَفْعَلْ ذَ‌ٰلِكَ يَلْقَ أَثَامًا  يُضَاعَفْ لَهُ الْعَذَابُ يَوْمَ الْقِيَامَةِ وَيَخْلُدْ فِيهِ مُهَانًا إِلَّا مَن تَابَ وَآمَنَ وَعَمِلَ عَمَلًا صَالِحًا فَأُولَـٰئِكَ يُبَدِّلُ اللَّهُ سَيِّئَاتِهِمْ حَسَنَاتٍ ۗ وَكَانَ اللَّهُ غَفُورًا رَّحِيمًا

And whoever does this shall be thrown the punishment. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace; Except those who repent and believe, and do righteous deeds; for those, Allah will change their sins into good deeds, and Allah is Oft-Forgiving, Most Merciful. (25:68-70)

7- It is a cause of [receiving] every good. The Most High says:

فَإِن تُبْتُمْ فَهُوَ خَيْرٌ لَّكُمْ

So if you repent, it is better for you. (9:3)

and The Most High says:

فَإِن يَتُوبُوا يَكُ خَيْرًا لَّهُمْ

If then they repent, it will be better for them. (9:74)

8- It is a cause of emaan and [receiving] a great reward. The Most High says:

إِلَّا الَّذِينَ تَابُوا وَأَصْلَحُوا وَاعْتَصَمُوا بِاللَّهِ وَأَخْلَصُوا دِينَهُمْ لِلَّهِ فَأُولَـٰئِكَ مَعَ الْمُؤْمِنِينَ ۖ وَسَوْفَ يُؤْتِ اللَّهُ الْمُؤْمِنِينَ أَجْرًا عَظِيمًا

Except those who repent, do righteous good deeds, hold fast to Allah, and purify their religion for Allah, then they will be with the believers. And Allah will grant the believers a great reward. (4:146)

9- It is a cause of the sending of blessings from the sky and an increase in strength. The Most High says:

وَيَا قَوْمِ اسْتَغْفِرُوا رَبَّكُمْ ثُمَّ تُوبُوا إِلَيْهِ يُرْسِلِ السَّمَاءَ عَلَيْكُم مِّدْرَارًا وَيَزِدْكُمْ قُوَّةً إِلَىٰ قُوَّتِكُمْ وَلَا تَتَوَلَّوْا مُجْرِمِينَ

“And O my people! Ask forgiveness of your Lord and then repent to Him, He will send you (from the sky) abundant rain, and add strength to your strength, so do not turn away as criminals. (11:52)

10-It is a cause of [receiving] the supplication of the Angels for those who repent. The Most High says:

الَّذِينَ يَحْمِلُونَ الْعَرْشَ وَمَنْ حَوْلَهُ يُسَبِّحُونَ بِحَمْدِ رَبِّهِمْ وَيُؤْمِنُونَ بِهِ وَيَسْتَغْفِرُونَ لِلَّذِينَ آمَنُوا رَبَّنَا وَسِعْتَ كُلَّ شَيْءٍ رَّحْمَةً وَعِلْمًا فَاغْفِرْ لِلَّذِينَ تَابُوا وَاتَّبَعُوا سَبِيلَكَ وَقِهِمْ عَذَابَ الْجَحِيمِ

Those (angels) who bear the Throne (of Allah) and those around it glorify the praises of their Lord, and believe in Him, and ask forgiveness for those who believe (saying): “Our Lord! You comprehend all things in mercy and knowledge, so forgive those who repent and follow Your Way, and save them from the torment of the blazing Fire! (40:7)

11- It is obeying the Will of Allah, azza wa jal. The Most High says:

وَاللَّهُ يُرِيدُ أَن يَتُوبَ عَلَيْكُمْ

And Allah desires that He should turn to you (in repentance). (4:27) So the one who repents is doing what Allah loves and Pleases Him.

12- Allah becomes happy with the repentance of His slave. The Prophet sal Allahu alayhi wa sallam said, “Truly, Allah is happier with the repentance of His slave than one of you who is on his mount, and upon his mount is his drink and food; then he loses his mount in the desert, and he searches for it until he loses hope; so he sleeps and then wakes up to find that his mount is beside him, and he says, ‘O’ Allah, you are my slave and I am your Lord’. He pronounced this mistake as a result of extreme happiness.” [Muslim]

taken from Tayyibaat