Why would ref be used for array parameters in C#?

Won't changes in the callee function be reflected in the caller function?

Changes to the contents of the array would be reflected in the caller method - but changes to the parameter itself wouldn't be. So for example:

public void Foo(int[] x)
{
    // The effect of this line is visible to the caller
    x[0] = 10;

    // This line is pointless
    x = new int[] { 20 };
}
...
int[] original = new int[10];
Foo(original);
Console.WriteLine(original[0]); // Prints 10

Now if we changed Foo to have a signature of:

public void Foo(ref int[] x)

and changed the calling code to:

Foo(ref original);

then it would print 20.

It's very important to understand the difference between a variable and the object that its value refers to - and likewise between modifying an object and modifying a variable.

See my article on parameter passing in C# for more information.