[css3-regions] What's the benefit of @region compared to plain selectors?

Hi,

I've tried to wrap my head around the concept of the
@region @-rule and failed to see the large advantage it gives over plain
old selector chains. To give an example:

@region #region-1 {
  p {
    color: pink;
  }
}

and

#region-1 p {
  color: pink;
}

seem identical to me, in that they style the potion of stuff
contained in the #region-1 element. I understand, that with the first
declaration you can address explicitly elements coming via a flow-into
source, while the latter addresses the original DOM elements before the
flow is calculated.

My question is, why do we need to separate
this? We already can address a single region's content with plain
selectors. The one use case I found in the list's archive was giving
proper ::first-line support to region's children. What speaks against
redefining ::first-line for region context, perhaps modified globally on
the flow source with a property:

article {
 
flow-into: main;
  flow-control: reevaluate-pseudo:
}

Cheers,
Manuel