Q. Isn't God the Father already Lord over all things? How can the Son also be King over all things?
A. The Lord Jesus Christ was given power and authority over all things upon the completion of his great mediatorial task, purchasing the salvation of His people by his atonement at the cross, and rising victoriously from the dead at his resurrection (Matt. 28.18). As a member of the Godhead, He already had the same power and authority as God; but in the economic and mediatorial relationship between Father and Son, the Father gave all such authority to the Son as a reward for the great work which he had accomplished.
Q. What difference does this doctrine make practically?
A. There is great practical benefit from rightly understanding that Christ is King over all things. In Ephesians, Paul teaches us that Christ is "Head over all things to the church" (Eph. 1.22). In other words, the purpose of Christ's kingship over all is to rule all things for the good of His people, that is, the church. We can rest confident in the knowledge that no-one and no thing can thwart Christ's purpose to sanctify, protect and prosper His church, even in the midst of an unbelieving and hostile world.
Q. Isn't Jesus Christ King over the church only? How can He be a kingly mediator over all people and things, when He is only mediator for the church?
A. It is true that Christ is a mediator between God and His people alone. But it is also true that to accomplish that great mediatorial purpose, the salvation of his people, the church, it was necessary that He be given all authority and power in heaven and in earth so that nothing could separate His people from Him. Hence, God the Father appointed the Son to be King over the entire universe.