what is difference between using as operator for casting IEnumerable to Array and using ToArray extension method of IEnumerable?

Let's say we have a variable of type IEnumerable<T> and we want to cast it into an array.
Resharper has suggested my a snippet like this:

var collection = input as T[] ?? input.ToArray()

Is there any chance in which input as T[] returns null so that the input.ToArray() part comes to action? Even if that occurs, why wouldn't we directly use the extension method instead of that? like this:

var collection = input.ToArray();

What is the difference between casting through calling ToArray extension method and using as operator?