All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, it is important for you to know that Istighfaar is asking for forgiveness, which is having one’s sins concealed (by Allah) and forgiven.
Repentance is to stop the sin and regret having done it.
Istighfaar may be mentioned alone without mentioning repentance along with it, and it may be mentioned along with repentance.
In the first case when it is mentioned alone, it means repentance and not just seeking forgiveness with the tongue while persisting in sin.
In the second case, in which it is mentioned along with repentance, the meaning of Istighfaar is asking for the forgiveness of past sins.
Ibn Al-Qayyim said in Madaarij As-Saalikeen:
“Istighfaar is of two types:
Stated alone and coupled with repentance. When it is stated alone, it is like the saying of Nooh to his people. Allah Says [on the tongue of Nooh] (what means): {And said, 'Ask forgiveness of your Lord. Indeed, He is ever a Perpetual Forgiver.} [Quran 71:10]
And also the sayiong of Saalih to his people, as Allah Says (what means): {Why do you not seek forgiveness of Allah that you may receive mercy?"} [Quran 71:10]
When it is mentioned along with repentance, it is like the Saying of Allah (which means): {And [saying], “Seek forgiveness of your Lord and repent to Him, [and] He will let you enjoy a good provision for a specified term and give every doer of favor his favor.} [Quran 11:3]
Therefore, Istighfaar alone is like repentance; rather, it is repentance itself and it includes asking forgiveness from Allah, which is asking Allah to wipe the sin, remove its effect, and protect one from its evil.
Hence, seeking forgiveness includes asking for protection, and it is this kind of (seeking) forgiveness that protects a person from punishment. Allah Says (what means): {…and Allah would not punish them while they seek forgiveness.} [Quran 8:33] Allah does not punish the one who seeks forgiveness.
As regards the one who persists on sinning and asks Allah for His Forgiveness, then this is absolutely not Istighfaar, and it does not prevent punishment.
Istighfaar includes repentance, and repentance includes Istighfaar. Both of them are interchangeable when they are mentioned separately. But when they are mentioned together, then forgiveness means: asking for protection from the evil of past sins; while repentance means: returning (to Allah in repentance), and asking for protection from the evil that one fears in the future from his evil sins.” [End of quote]
Shaykh Ibn ' Uthaymeen was asked about the difference between repentance and Istighfaar, and does Istighfaar have an effect in decreasing sin while persisting on it?
He replied:
“Istighfaar is about a past sin, and repentance is for the future (i.e. future sins); Istighfaar is an introduction to repentance, such as a starter before the dessert; it is for this reason that Allah mentioned them together, as in His Saying (which means): {And [saying], "Seek forgiveness of your Lord and repent to Him.} [Quran 11:3] However, Istighfaar while insisting (on the sin) is of no use, because this is closer to mocking than to being a good deed.” [End of quote]
Istighfaar that was reported about the Prophet and that he was seeking forgiveness in the day and night seventy times or more, was coupled with repentance. Abu Hurayrah narrated that he heard the Prophet saying: “I swear by Allah; verily, I seek the forgiveness of Allah and I repent to Him more than seventy times daily.” [Al-Bukhari]
Thus, it becoimes clear to you that Istighfaar which is mentioned in Chapter Nooh [Quran 71] and which leads to an abundant provision, having many children, and much wealth is not verbal only. This also applies to Istighfaar which is mentioned in the verse (which means): {But Allah would not punish them while you, [O Muhammad], are among them, and Allah would not punish them while they seek forgiveness.} [Quran 8:33], as in the abovementioned statement of Ibn Al-Qayyim .
As for the benefits of seeking forgiveness, then they are so many and well-known. Whoever reads the Book of Allah Almighty or reads the books of the Sunnah, he will find many of them.
Allah knows best.