I'm not entirely sure if it's *the* reason, but 
I've noticed that when one puts cels into the 
bags, one often gets some air trapped in the bag, 
which makes the bag bulge out a bit.  When the 
corner of the bag is snipped, it lets the  air 
out, so that the cel can be stored more snuggly 
in the bag, and more easily packaged for shipping.
It's a theory, anyway.

~Mala (who should probably be asleep)