Have you seen an ant farm? It's a glass fish tank filled with dirt, and it has a colony of ants in it. Sometimes they dig their tunnels along the glass, and you can watch them working away.
Earth is God's humanity farm, just to see what humans will do next. There is plenty of room in the world. What we are missing is wisdom among ourselves to use it properly.
Usually ant farms containers are built with little thickness on purpose, so children are more likely to see how ants work in the tunnels, just a little detail I wanted to point out, as I was myself interested in antfarms when I was a child.
In the case of humanity being God's ant farm, I like to think God creates us beyond being a source of amusement for him, in the same way an ant colony is a source of amusement for a child. We are supposed to have a purpose beyond that, being granted by him to us.
It is true there is plenty of land and places to be inhabited in this planet, but yet, we are seeking to build on the moon and on the red planet as soon as possible. It must be because something has changed.