Having a powerful ministry means knowing the limitations of our power to change the world.
Jesus talks here and says that the kingdom of God will grow (an endeavor many of us desire to have) not because we have made it grown but because the author of life is at work in making it grow.
Our job is not to make it grow, but to partner with the Author of Life to see that task accomplished in our planting of seeds.
I think this has something to say for people in ministry who stress out so much about the people around them not changing in the way and in the timing that they anticipated. Or for people who have a controlling spirit about them to try to manufacture kingdom movements in their midst.
This passage is for our comfort and for our reality check–God does the work, so take it easy!