The Kingdom of God matters because everything in life comes out of the Kingdom of God, and everything will eventually end, except for the Kingdom of God.
I think of the Kingdom of God like the axle of a bicycle wheel. The wheel was created for a purpose to take the rider from here to there, but it cannot fulfill its purpose unless the axle is in the center. Everything revolves around and finds its meaning around the axle. The Axle is the Kingdom of God.
But seek first the kingdom of God and his righteousness, and all these things will be added to you. Matthew 6:33
We can ignore the Kingdom of God and believe that we are the center of our world, for a limited time, but ultimately we’ll find that life is meaningless and empty without the King in his rightful place. When all is said and done on earth, all that will be left is the Kingdom of God.
As Solomon once said, “Vanity of vanities, … vanities of vanities! All is vanity. What does man gain by all the toil at which he toils under the sun? A generation goes, and a generation comes …” Ecclesiastes 1:2-4a