When to use OrderByCompletion (Jon Skeet) vs Parallel.ForEach with async delegates

Recently Jon Skeet at NDC London spoke about C# 5 async/await and presented the idea of "ordering by completion" a list of async tasks. A link http://msmvps.com/blogs/jon_skeet/archive/2012/01/16/eduasync-part-19-ordering-by-completion-ahead-of-time.aspx

I am a bit confused or should I say I am not sure when will this technique be more appropriate to use.

I cannot understand the difference between this and the below example

var bag = new ConcurrentBag<object>();
Parallel.ForEach(myCollection, async item =>
{
  // some pre stuff
  var response = await GetData(item);
  bag.Add(response);
  // some post stuff
}

or ForEachAsync as explained by Stephen Toub - http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx

EDIT: Found a blog post from Stephen Toub explaining "Ordering by completion" <=> "Processing tasks as they complete". Worth reading. After reading this I could clearly understand the reasons how it works and also when to use this technique.