To a lot of people, it justifies paying a little 
extra for an expensive cel.  For AIC cels in 
particular, it may also significantly raise the 
value of certain cels.

I usually don't go searching for sketches of my 
cels, but I did find extra gengas to go along 
with a cel of mine (which I haven't received yet, 
though; it's on my incoming cels page).  I got 
the cel off Yahoo Japan, and the gengas off eBay.