Joab calls on David to finish the work that he had started in capturing the enemy territory, he does so in order that the glory might rest upon the king, and not in himself. This is how we as Christians ought to be in service to God, we need God to finish the work through us, that He May receive the glory, our job as servants of God are to honor and glorify Him. Just as Jesus is the author and the FINISHER of our faith, so to did joab seek David to finish the work of taking the cities of the ammonites. When we do things for God we do so not for ourselves or our own glory, but that all honor and glory point back to him, we are to advance the kingdom of God here on earth, and in order to do so we must focus on Him and glorifying His name, seek to be a humble servant showing love to your neighbors and glorify God in all you do.
Amen