Fundamental question about circuits and grounding
Your question about protection is a very important one. Every electrician, technician working with heavy machinery, and electrical engineer should know how these things work.

All About Circuits published a very comprehensive online compendium about electronics and electrical engineering. For your question, I recommend reading this article about safety in circuit design.



In short, current chooses the shortest path to a lower potential. Without a grounded metal case, your body may become a path to a lower potential (earth). Your body may not be the best conductor, having e.g. 1000 Ohms, but it's still many orders of magnitude less than air. 

Your kitchen floor is approximately earth potential. Buildings, soil, etc are not very good conductors, but they still have a low resistance compared to, say, air. 
Earth's soil may have a resistance of a few Ohms, maybe tens of Ohms, it depends on many factors. What's certain is that it does conduct.

Current choses the shortest paths. What's more, a large contact surface will create more paths for the current to flow, thus lowering the resistance of something which at first glance is not a good conductor.