Why C# local variables must be initialized?

Question

I am reading MCTS Self Paced Training Kit (70-536) Edition 2 and in the 1st chapter we have the following.

How to Declare a Value Type Variable To

Answer 1

A variable in a method (method scope) needs to be initialized explicitly. A variable (or 'field') at class level is initialized automatically with the default value.

class Test
{
   bool b; // =false
   int i; // =0
}