Why is GDP growth so important that central banks are trying to impose negative interest rate?
Central banks do not try to increase growth. Instead, they commonly have two targets: 1) to maintain price stability and 2) to minimize the output gap. Let us ignore the first goal for a moment. The minimization of the output gap implies that if output falls short of what the central bank believes to be the natural level of output, then it will try to use economic policies which promote growth.

In your example, 10 units of bread being produced seems to be the natural level of output. If the central bank sees that the economy produces only 9 units of bread, it will try to stimulate the economy with low interest rates. If it observes 11 units being produced, it will raise interest rates to reduce the output. (Note that this is still possible even if firms produce 9 units of bread using 10 (or even 100) units of labor, i.e. if firms are not cutting employees efficiently.). Thus, if the central bank of Japan currently has low interest rates, it believes that current output falls short of the natural level of output, despite low unemployment rates.

You may still ask why the central bank tries to minimize the output gap in the first place. Why should it not just focus on price stability or why not even do something random. It turns out that up to second order approximations, an interest rate policy which minimizes a combination of price volatility and output gap volatility maximizes people's welfare in so-called new-Keynesian models. This was first shown by Woodford and Rotemberg in several articles:


  Woodford, Michael. "Inflation stabilization and welfare." B.E. Journal of Macroeconomics, 2, (2002).
  
  Woodford, M. (1999). Optimal Monetary Policy Inertia. Retrieved from http://www.nber.org/papers/w7261
  
  Rotemberg, J., &amp; Woodford, M. (1999). Interest rate rules in an estimated sticky price model. Monetary Policy Rules. Retrieved from http://www.nber.org/chapters/c7414.pdf