What is the advantage in using placeholders vs using the variable?

Answer 1

It's prettier :)

Console.WriteLine("Hello " + userName + ", I am " + AIName + ". I wad created " + days + " days ago.");

Console.WriteLine("Hello {0}, I am {1}. I was created {2} days ago.", userName, AIName, days);

Answer 2

it increases the readability of your code, especially if you have long variable names.