it is beyond any doubt in our hearts that Allah the mighty and sublime knows everything, from eternity to eternity. with that firm conviction let us remember these verses from the quran.
- And I did not create the jinn and mankind except to worship Me. [51:56]
after giving us a purpose in life, Allah the exalted and glorified says,
- And your Lord says, "Call upon Me; I will respond to you." Indeed, those who disdain My worship will enter Hell [rendered] contemptible. [40:60]
we can see the word 'worship' in these two verses. now lets understand this from the sunnah of our beloved Prophet (S.A.W),
It was narrated from an-Nu‘maan ibn Basheer (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Du‘aa’ is worship.” Then he recited the verse (interpretation of the meaning): “And your Lord says, ‘Call upon Me; I will respond to you.’ Indeed, those who disdain My worship will enter Hell [rendered] contemptible.” [Ghaafir 40:60]. Narrated by Ahmad in al-Musnad (18352) and al-Bukhaari in al-Adab al-Mufrad (714).
just to understand it better, ill quote from our scholars as well.
At-Teebi said in Sharh al-Mishkaat (5/1708): The grammatical structure of the sentence, in which the word al-‘ibaadah (worship) appears with the definite article al- is indicative of exclusivity, and that worship is nothing but du‘aa’ [supplication].
from this we can understand that dua'a is a form of worship. in order to fully comprehend the power of du'a and the importance of it in the lives of muslims, the following extract is very useful.
You should understand that the original linguistic meaning of du‘aa’ is to ask or seek, so it is asking for what the supplicant wants to get and prefers to attain. Sometimes it is the du‘aa’ of asking of Allah, may He be glorified and exalted, and beseeching Him, such as when the person says “O Allah, forgive me; O Allah, have mercy on me.” And sometimes du‘aa’ is by taking appropriate measures by means of which one may attain what one wants, which is focusing on obeying and worshipping Allah, and remembering Him, and focusing on doing what He loves His slaves to do. This is the true essence of faith. [Ibn Rajab in Fath al-Baari (1/20)]
now that the importance of making dua'a is understood, let us remind ourselves of the beloved sunnah of our prophet (S.A.W). there are many authentic narrations about making dua'a and insha Allah, these ahadith will clarify even more why we make dua'a even when everything is already written. (i believe understanding 'Qadr' in it's whole concept deserves a dedicated question)
Anas ibn Malik reported: The Messenger of Allah, peace and blessings be upon him, said, “I am amazed by the believer. Verily, Allah does not decree anything for the believer except what is good for him.”[Musnad Aḥmad 12495][Sahih (authentic) according to Al-Albani]
with a firm belief in that, remind ourselves of this next hadith.
“Nothing can change the Divine decree except dua”.[Musnad Ahmad, 5/677; Ibn Majah, 90; Jami` Al-Tirmidhi, 139. Classed as hasan by Albani]
in another narration in the same context,
“Nothing increases one’s life-span except good deeds, and nothing repels Divine Decree except du’a. And very, a person may be deprived for sustenance due to a sin that he does!” [Narrated by Ibn Majah #90 and confirmed authentic by Shaykh al-Albani in as-Sahihah #154]
a corroborating narration,
- The Prophet, upon whom be peace, said:
“There is no Muslim on the face of the earth that asks Allah for anything except that Allah gives it to him, or averts from him a similar evil, as long as he does not ask for something evil or for breaking the ties of kinship.” [Authentic, narrated by at-Tirmidhi from ‘Ubadah ibn Samit, as is mentioned in Sahih al-Jami’#5637]
and i'll mention one more to establish more emphasis on why do we make dua'a,
It was narrated from Ibn ‘Umar (may Allaah be pleased with him) that the Prophet (Peace be upon him) said: “Du’aa’ may be of benefit with regard to what has already happened or what has not yet happened, so adhere to Du’aa’, O slaves of Allaah.” [Narrated by al-Tirmidhi (3548)]
from these few verses and ahadith we can see and understand why it is so important to make dua'a regardless of what has been written for us. never despair the mercy of Allah.
may Allah grant you mercy and guide you to the straight path.
Allah knows best.