By default, method arguments are passed by value. That is, arguments are copied and passed into the method. Therefore, modification to the argument inside the method body does not affect the original value. In most of the cases, modifications are unnecessary.
Other programming languages, such as C++, has a const parameter or similar concept: This indicates that the parameter inside the method body is a constant that cannot be reassigned. It helps to avoid mistakes where you unintentionally reassign a method parameter in the body, and improves the performance by disallowing the unnecessary assignments.
C# 7.2 introduces the in parameter (aka. readonly ref parameter.) A method parameter with in modifier means that this parameter is by ref and read only within the method body.