Sir, it's about using your head and at times understanding that some things are just beyond your understanding. However, on why we spread the gospel is not one of those things that are above our understanding. God does not tell you to do something in vain. He has a reason for it. Spreading the Gospel, as we are instructed, is obvious. To lead people to His Son, the Lord Jesus Christ.
God did not command us to preach to an apple tree because he has no reason for us to do it. He commands us to do things out of a purpose. He is not vain.
So you're right. You do what God tells you, but God reveals his character in the Bible. He isn't vain. Spreading the Gospel is to save people from damnation and to bring them to his glory and everlasting life in fellowship with the one true God.
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I thank God that he allows me to have a purpose and reason .. a job and an assignment within his kingdom. God could do everything for us, but allowing me the wonderful opportunity to serve him is from his grace and blessing. I want to be able to serve his kingdom, that is what our existence is about, serving God, loving him, enjoying all the good things of God, being a family, and growing that family.