Chip, as usual, has eloquently stated the heart 
of the matter.  I hadn't thought about cel 
collecting in quite those terms before--as a 
continuum of shared experience--but he is exactly 
right.  My own explanation will sound slightly 
different, and is highly personal, but when 
boiled down to its essence is the same as his.

I've spent a lifetime marvelling over our 
species' ability to tell stories.  We tell 
stories through music, art, sculpture, 
literature, religion, science -- so many ways!  

Beginning with Grimm's Fairy Tales when I was old 
enough to read them, plunked in front of a TV 
tuned to old PBS broadcasts of Wagner operas, I 
have randomly soaked up mythology, folklore, 
religion and cultural tidbits from different 
times and places.  As I went through my formal 
schooling I tended toward the same types of 
material -- participation in choral music groups, 
studying bits and pieces of Greek and Roman 
classics, mythology, the history of the English 
language, comparative religion, and 
fantasy/science fiction.  

Fast forward 20 years to my first experience with 
anime.  It felt like stepping into a perfect 
mixture of everything I've ever found important.  
Accessible, wonderful music, both vocal and 
instrumental. Brilliant colors in a 
highly "readable" visual shorthand that 
nevertheless has the ability to convey tremendous 
depths of experience and emotion.  And the 
inclusion, either musically, visually, or 
verbally, of the touch of a very long and complex 
history of beliefs and meanings.  If you've never 
had a chance to see Crispin Freeman's film on the 
Mythology of Anime, try to.  He's another person 
who has put words to the heart of the matter.

And now for the other explanation, which my 
studies into how people learn have made necessary 
("you're a doctoral student who watches CARTOONS? 
Why?")  

Our brains are wired to learn by touch, sound, 
words, and visual and emotional experience. 
Watching anime reaches almost all of those 
channels of learning at the same time -- and it 
doesn't matter whether you're watching an 
Evangelion or a complete piece of fluff.  It 
triggers memories and creates new connections 
among bits of shared knowledge and experience 
from many lifetimes, in a way that single-medium 
storytelling just can't.

The thing about cels, figures, pins, posters and 
plushies, however, is that they add the tactile 
to the entirety of the anime learning experience. 
As Chip said, there is something about holding in 
your hand a physical part of the experience that 
can't be matched in any other way.  Likewise 
figures.  To hold and look at the magnificent 
detail in these miniatures of what was originally 
a two-dimensional rendering is to experience the 
completeness of the human experience.  We have 
the ability to use symbols to recreate for each 
other an entire universe of experience.

Whee! and of course, collecting cels is plain 
fun. 

Do I have a favorite cel? not really.  But this 
one is one of those that doesn't even need to be 
placed in context to carry an amazing depth of 
meaning.