Separate functions into validation and implementation? Why?

I was reading a C# book in which the author (some dude named Jon Skeet) implements a Where function like

public static IEnumerable<T> Where<T> ( this IEnumerable<T> source, Funct<T,bool> predicate ) 
{
    if ( source == null || predicate == null ) 
    {
        throw new ArgumentNullException();
    }
    return WhereImpl(source, predicate);
}

public static IEnumerable<T> WhereImpl<T> ( IEnumerable <T> source, Func<T,bool> predicate ) 
{
    foreach ( T item in source ) 
    {
      if ( predicate(item) )  
      {
         yield return item;
      }
    }

}

Now, I fully understand how this works and that it's equivalent to

public static IEnumerable<T> Where<T> ( this IEnumerable<T> source, Funct<T,bool> predicate ) 
{
    if ( source == null || predicate == null ) 
    {
        throw new ArgumentNullException();
    }
    foreach ( T item in source ) 
    {
      if ( predicate(item) )  
      {
         yield return item;
      }
    }
}

which brings up the question of why would one separate these into 2 functions given that there would be memory/time overhead and of course more code. I always validate parameters and if I start writing like this example then I'll be writing twice as much code. Is there some school of thought which holds that validation and implementation should be separate functions?