Allah is almighty and all knowing, he knows about our hopes and wishes and about our destiny, he knows what's best - so surely dua can't be about influencing him or making him change his mind. But then what is dua all about?
Abu Ayyub Ansari reported that Allah's Messenger (may peace be upon him) said: If you were not to commit sins, Allah would have swept you out of existence and would have replaced you by another people who have committed sin, and then asked forgiveness from Allah, and He would have granted them pardon. (Sahih Muslim: bk. 37, no. 6621-6622, Siddique).
The true purpose of Du'a as said above is similar to Salah, the way by which we remember Allah.
Generally, Du'a is something which we ask from Allah. Usually, the prophets have been recorded asking in the Qur'an to Allah, about some or other thing, expecting help or forgiveness from Allah.
Du'a is not about this present life or destiny in this world.
Whosoever desires (with his deeds) the reward of the Hereafter, We give him increase in his reward, and whosoever desires the reward of this world (with his deeds), We give him thereof (what is written for him), and he has no portion in the Hereafter. (Qur'an 42:20)
May the creator guide us all.
http://fatwa.islamweb.net/fatwa/index.php?page=showfatwa&Option=FatwaId&Id=69140 dua is ebada same to salah .god knows what we need but as he loves us he want to see us asking .it's like seeing someone you love asking you somthing that you allredy know he want's it.also dua if god doesn't give you what you are asking he might get somthing harmful out of your way insted or it will be saved for you to after life