I think eyebrow windows are actually a relic from the past. Used to fly the DC-8 with quite big ones, usually stuffed with something to keep the sun out like others have written here. Same thing on the B737s I flew, tailored sunshades were some help though. The original 707 had them (same nose section as the 727,737 therefore...) but I always guessed that was some kind of military relic due to Boeings planned (and successful) military use of 707 derivatives. Perhaps former military guys know? Something originally to do with formation flying? Or as suggested here tight circuits?