Different people will probably say different 
things....

Cels are a palpable, physical, visually-
immediate, and causal connection to shared 
intellectual and emotional experience with 
important intimates.

In other words, I and mine watch Anime shows 
together, and it is an important shared 
experience for us.  We discuss the shows, 
sometimes we get choked up about the shows, and 
the shared Anime viewing and our reactions to it 
are a part of our shared experiences as people.  
References to good-quality Anime make up part of 
the verbal shorthand that we use with to discuss 
with one another life and our struggles with life.

What really motivates the collecting of cels in 
this house is to be able to hold in your hand a 
palpable, physical part of the creation of that 
experience;  and to be able to look at the cel 
and understand that the shared Anime experience 
was a result of someone painstakingly crafting 
that cel by hand;  and to know that there was a 
cause-and-effect relationship between the piece 
of plastic in your hand and the shared 
experiences that we shared with one another.

The cels are also pretty things to look at in 
their own right, and capture the artistic 
sensibilities of the character designer and the 
director.  When rational people enjoy a work of 
art (which Anime is arguably a form of), they 
enjoy looking deeper into the foundations and 
creative process of that art, and cels are a way 
to dig into the foundations and creative process 
of an art form that we enjoy.  And so there is 
happiness in looking at the pretty paintings; in 
this sense, it's actually not too different from 
collecting all of Monet's series of paintings of 
Wheatstacks -- just a lot less expensive!!

Cels might be a good investment, but that factor 
is really a rationalization for the genuine 
reasons.