Why make Dua when all matters are decreed?

Many of us hold the view that why make dua to Allah, when He has already preordained all matters? Withholding dua with this idea (or any other) in mind is a serious mistake and we would be missing on the blessings of Allah.
It was narrated from Ibn ‘Umar (RA) that the Prophet (SAW) said:
“Dua may be of benefit with regard to what has already happened or what has not yet happened, so adhere to dua, O slaves of Allah.”
[Narrated by Al-Tirmidhi, 3548 and classed as hasan by Al-Albaani in Saheeh Al-Jaami’, 3409]
In a hadith it says:
“The decree cannot be overturned except by dua.”
[Fataawa Noor ‘ala al-Darb by Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him)]
Shaykh Al-Islam Ibn Taymiyah said in Majmoo’ Al-Fataawa [8/69]:
If a person says that he does not say dua or ask of Allah, because he relies on the divine decree, he is also erring, because Allah has made dua and asking means of attaining His forgiveness, mercy, guidance, support and provision. If good is decreed for a person he will attain by means of dua what he cannot attain without dua. What Allah has decreed and knows with regard to His slaves’ circumstances and destinies is only decreed on the basis of means, and decrees will be fulfilled at the appointed times. There is everything that happens in this world or in the Hereafter happens on the basis of cause and effect; Allah is the Creator of both cause and effect.
Disregarding the principle of cause and effect is contrary to reason.
Shaykh Ibn ‘Uthaymeen said in Al-Majmoo’ Al-Thameen min Fataawa Fadeelat Al-Shaykh Muhammad ibn Saalih Al-‘Uthaymeen [1/157]:
Dua is one of the means by which what is asked for is attained. In fact it wards off the divine decree and nothing wards off the divine decree except dua, and that may happen in two ways. For example, a sick person may pray to Allah for healing and be healed, and were it not for the dua he would have remained sick, but he was healed by means of the dua. But we say: Allah decreed that the sick person would be healed from this sickness by means of dua, and this is what was decreed. He thinks that were it not for the dua he would have remained sick, but in fact he did not ward off the divine decree, because the basic principle is that dua was decreed and that the healing would come by means of this dua, and this is the original decree that was decreed from eternity. The same applies to everything that is decreed to happen through some means or cause. Allah has made this means the cause for the thing to happen, and that was decreed from eternity, before it happened.