It confuses, why we have to convert number input into integer when it is already int?

Answer 1

That's because, when you use Console.ReadLine(), it accepts a String. If you do not convert to Int or Double, it'll produce errors.

Answer 2

in c# user input like a textbox are usually string, so you have to convert them to int or double to use.
C# wont convert anything itself untill you say!

Answer 3

When using C# you have to be specific when programming, that's why you have to convert number input into an integer when it is already. Also, that goes the same for data types, functions(you will learn that in later ;), etc.