Warm Standby question

Hi,

(Note: I have never used log shipping before, I'm just interested in the
concepts, so I'm might be missing a very important aspect)

I was reading the blog entry about HA and warm standby:
http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-point-in-time.html

The image that explained how log shipping works, strikes me as being a bit too
complex.
<http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv-w/s1600-h/pg_warm_standby.png>

According to the picture it basically works like this:

Master -> Copy master archive directory -> Copy to standby archive dir -> copy
to pg_xlogs.

When I look at this chain I'm asking myself, why do I need the two archive
directories?

Why can't the master copy the files directly into the pg_xlogs directory of the
standby server?

Thanks
Thomas