How are we of use to God? This is an important question, an essential question, that must be answered. When we help man are we being of use to God?
I think that the use Chambers is referring to, is a spiritual one, one that has eternal consequences. The call of a Christian is to make disciples and to be His witnesses. God uses us to make a difference in someone's life that last. I am thinking mainly of salvation, but there are other things as well. God has used many people in my life to inspire me to walk closer with God. But that's it, it's about God in the end.
It is entirely possibly to help man without having been of use. That is, if you define being of use in terms of God and eternity. Me helping someone out by giving them a ride, does not mean I am of use to God in that situation. It is a good thing to help others. We are told to help others. But helping and being of use are two distinct things. God can use the times we help man, but he doesn't have to.
I find this to be humbling. I can do many great things to help man, but they could all be for naught if I don't have the right relationship with Christ. God decides how to use us, we don't.