IProcessor Process() signature

Is there a reason why Output is supplied as an argument into the Process function - why not just let the developer of the function create an instance of it themselves within the function? Then the function would be simply i.e. IRow -> IRow.

I know that this is a somewhat similar pattern to the old HDInsight C# SDK, which took in the input stream and output stream, and you manually "pushed" data into the output stream, but here we're just returning some IRow anyway.

Is there some state that is set in advance on the Output argument or something?