Why for print the value of variable is necessary "... {0}..."?

Question

sorry for my English

Answer 1

It isn't necessary, it's just one of the ways of doing it.
I assume you're talking about Console.WriteLine?

In the following code:

string x = "This";
string y = "is";
string z = "an";
Console.WriteLine("{0} {1} {2} example", x, y, z);

The output will be "This is an example", because {0} refers to the first given variable (x), {1} to the second one (y) and {2} to the third one (z).

But another way of doing the same thing would be:

Console.WriteLine(x + " " + y + " " + z + " example");

Answer 2

Or, in case you are wondering why 0 (instead of 1), such things are generally counted starting from 0 in C#.
You'll see the same thing happening when you study arrays.