1) The sun coming through the clouds is somewhat painful and migraine inducing for me. I just know I'm more likely to squint, and squinting causes headaches (and wrinkles...)
2) Sunglasses provide extra SPF for my delicate eye area. As long as you pick ones with UVA/UVB protection, you're doing that easily damaged, thin eye area skin a favor. Ditto for why I like huge-cover-half-your-face windshield style sunglasses. SPF FTW.
3) They make an outfit look like an outfit. I'm not sure how cute this outfit would be without these gigantic, oversized aviators.
4) They let you blatantly people watch strangers while feeling like they can't see you staring. (Realistically they probably could, if they would look up from their smart phones long enough to notice!)
5) I love sunglasses so much, and I'm not sure I could justify owning as many as I do if I cut them out of the fashion equation on overcast days.
6) You can wear them instead of makeup when you run errands.
7) They protect your delicate eye area from wind burn and wind related debris.
8) Did you ever watch the TV Show La Femme Nikita? Not the Maggie Q one but the old one on USA with the blonde? She had an apartment filled with strings, and off those strings hung hundreds of sunglasses. She was SO. COOL. And I have never been able to get this image out of my head.
9) In a pinch they can act as a headband (although I hate how that stretches them out.
10) A pink/rose/amber lens designed for an overcast day can actually improve visibility! So basically these, or the classic rose aviators are just like, functional...