The question why do we exist remain always a question for human understanding. You should understand that there is no perfect creation neither there is no perfect destruction. Creation and destruction are part and alwaysgoing on continuously. When I am writing this there is some universe which is created and some which is being destroyed. There are many universes which are destroyed and created. So there are always three things present, 1) Brahman i.e God. 2) Atman i.e true self. 3) Maya or prakirit or Nature i.e Matter or energy both are interconvertible to each other. Brahman by use of Maya, created universe and when purpose is over universe and destroyed and again new universe is madeby Maya. As Shri Krishna says in Bhagwat Gita ""Never was there a time when I did not exist, nor you, nor all these kings; nor in the future shall any of us cease to be. As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. A self-realized soul is not bewildered by such a change." ( Bhagavad-Gita 2.12-13)" God is truth & truth is always having existence. God was there when we all were not, God is there when we are having our present existence & God shall be there even when we shall not be there. He is the ultimate truth & truth exists always. In the night you see many stars in the sky but when the sun rises in the morning you do not see these stars at all. Now can you say that there is no existence of stars, as these shall be again visible, once the night comes & whole sky shall again lit with stars proving their existence. Similarly when our mind wanders in ignorance, during that time we do not believe in His existence but when our mind is lit with the light ofknowledge, we are able to see the existence of God. Then we do not say that there is no existence of God. "Sri Krishna said: O Arjuna, I am the generating seed of all existences. There is no being—moving ornon-moving—that can exist without Me" (Bhagwat Gita: Chapter Ten verse 39) So direct answer to your question is "purpose" i.e we exist for some purpose. Purpose is broad term it is different fordifferent human or animals or plants. A whole life can be spent for searching the purpose of life. So search your purpose of life. God have also purpose he creates, sustain and destroy whole universe. He alone maintains this cycle of creation and desruction. That why we says Brahman have three Gunas or aspects i.e Brahma, Vishnu and Mahesh. Creator, sustainer and Destroyer. Now question arises why there are different species? different types of mankind? Why God created us ? God create us so that one can enjoy his/her results of Karma. These results can be good or bad according to karma. to know more about these kind of question please go thru this link-->> Why God created universe? What is solution of life Query: If we wont have exist,what purpose the god being supreme power have served.? Answer: As it known to you, you are exist before time and will be after time. The real question is why we attain this body? As you know Atman or true self is cant do karma or we can say Atman is not attached to karma, To attain the truth and to enjoy the fruit of Karma we attain body or God let us to attain this body which is made by matter or Maya. Atman cant do any Karma or neither atman can enjoy any kind of fruit. This fruit is called as Raas, and we are all part of Raasleela of God in this world. Or we can say we are Rasik i.e thirsty of Raas. So it cant happened that we wont exist. may be we wont exist on this realm we will present of other realem, May be not in this time may be on other time, It is not a matter that What God will do if we are not present. God do according to his own nature.