Why is it necessary for spirit-lives to come back to earth?

People frequently ask why it is necessary, for a spirit to come back to earth, when it could progress in the spirit world. 

The whole point is this—

It is not the spirit that has to learn to progress through contact with the physical—

It is the personality. 

In the spirit world, you progress spiritually, but you cannot develop personality there. 

Earth's lessons are necessary for this, for the spirit develops personality through the senses of the physical body by using them appropriately and overcoming the temptations of those senses—and so learning self-restraint.