What are the pocket/flap on dog's ears?

[HeatherAnn]

I'm house/dog sitting for my Aunty this week as I have nothing better to do with my days and her dog (who looks like my dog if he was shaved) has a "pocket" on her ear. I've seen a lot of dogs and cats with it and was wondering if you could enlighten me as to what it is? sorry for being a numptie

 

[StableMum]

They are called Henry's pockets. I 've no idea why or whether they have any use. Seem to be a good place for harvest mites to lurk though.