Critics today generally look at this book as a text about colonialism and industrialization. They see Robinson Crusoe as a symbol of those things as they go out and dominate the primitive world.
However, I think that Defoe probably wrote the book in part as a religious thing. I think that he wanted to use the book to help inspire people to put their faith in God.
One of the major themes in the book is how Robinson comes to God -- how he comes to have faith in God for the first time in his life. The book explicitly tells us that we need to realize that God has blessed us in many ways. Robinson comes to believe that he has been put through all his ordeals by God's will. He think God is trying to send him a message that he does not fully embrace until he has been on the island for quite some time.
I think Defoe hoped the book would inspire people to turn to God.
Please follow the link for a detailed discussion of this topic.