Why does the power company provide a neutral line?
A voltage is a potential difference between conductors. For instance the difference between the live and the neutral wire. 

The power company delivers these two lines to ensure the potential difference to the customers. If it would only supply the live wire and the ground of a local customer is used, it's unknown which voltage the customer receives. 

It is because there is no knowledge about the potential difference of neutral between the power facility and your house. It can be higher or lower. And then the resulting voltage on your devices can also be higher or lower.