What are those little flaps of skin for behind their ears?
For receiving intelligence and directives from their home planet.
Some cats have them on their eyes as well. They are called epicatthic folds.
Are you talking about what I think you’re talking about? A pouch-like structure on the edge of the ear? If that’s not what you’re talking about, you can ignore what follows. I couldn’t find an anatomical atlas of the cat (“map of a cat” as R. P. Feynman would say) that considers this important enough to mention, but dogs appear to have more or less the same thing. The book Canine Anatomy: A systematic Study by Donald R. Adams (1986) says (pp 12-14),
If the anatomist doesn’t know what it’s for, I’m not going to speculate. I have scanned the accompanying diagram and you can see it here. I have underlined “cutaneous marginal sac” in red, since it’s hard to read.
It’s for storing mice to eat in the winter.
I wonder about this question every time I play with that cute little extra-soft flap on my own cats’ ears, and I don’t know the answer, but Labradorian, you cracked me up! 
LOL
The only remotely plausible explanation I’ve been able to think of is that it contributes to the ear’s flexibility and freedom of movement, since cats’ ears have a VERY wide range of motion - from pointed straight forward and downward, to 180 degrees back and completely flat against their heads. Kind of like how you have loose skin at your elbow to allow for the flexing and stretching.
Now, as to why there’s a little nick/slot in the front side of the flap…