Usually the reason the sketches are turned over on
the back of a cel is that they have become stuck
to the paint of the cel.  Most of the people who
had them previously did not care or did not know
that this would happen.  

I personally like it when my sketches are seperate
from the cel, but I can't complain if they are
stuck facing out from the cel.  I really hate to
see really nice sketches that are stuck to the
back of the cel facing toward the paint because
you can usually never remove them without damaging
one or the other.  

I believe the sketches are drawn first and then
used to help paint the image onto the cel. 
Usually they contain words on them and
descriptions or different colors to tell the
artists what color goes where.  They also put a
sequence number on the upper corners of the cel
and the sketch so that they can tell where the cel
will go.  

I'm not an expert but there are two different
types of drawings that you usually see for anime.
 Mainly you see the sketches of cels which I
believe are called Douga, but there's another
drawing called Genga and I don't think it is used
as part of the cel its just a main drawing and
they are harder to find and worth more.

I personally enjoy the sketches just as much as
the cels :)
I have lots of douga at my site, and I'm fixing to
put up a gallery soon dedicated to just it becuase
I have a few sets of very nice drawings.