You're working alone without being alone. It is mentally stimulating. It can be and feel safer than a library if you're a grad student. And to some earlier points, I know it's not enough to justify the hours, but I spent a small fortune at those places: I wouldn't be there if I wasn't ordering. And I would become friendly with the baristas: one of us might be having a bad day and share a laugh and it helped. So that human element is a comfort when you are devoted to a long, solitary endeavor.