The mansions spoken of by Jesus in the Scriptures represents the place of reward in the domain of the Father our Creator. He is preparing his students for a life of labor and eternal reward.
This reward is not fleeting, like all rewards in this physical world, but it is talking about an eternal reward which can be enjoyed forever. This is the place that Jesus wants us all to go to... The mansions in the celestial world, and what we do that has eternal reward here will have a bearing in the eternal plane.
Now, these rewards are not just to acquire something to enjoy forever. A new Jerusalem; a new world will come. Previously, it was 'Eden' for us who are in this wave of human life. But for those after this world, it will be 'New Jerusalem'. The rewards, attainments and force we employed in helping Jesus harvest souls of Light will and can be used to help the inhabitants of the coming wave of sentient beings in the next world. This is the value of these attainments which Jesus wants his disciples to recieve.
We have a lot more work in the next wave than there is in this wave of human existence, and we ought to labor excessively for our eternal reward. Why do you think there are Angels and Archangels? We must work and obtain eternal rewards that can be used to serve others for God's sake in the next world, and then the next world, and then the next one... For all eternity.
"Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened."