Manga cafés are places where, for a nominal fee, you get access to a huge number (often thousands) of Japanese manga and can spend some quality leisure time with the manga of your choice. They are also used as a cheaper alternative to hotels, when people get stuck due to typhoons etc.
Besides they are open the whole night. Some people kinda live there. It's cheaper than to get a hotel room.
Oh my God! Manga and snacks? Sign me up immediately! I hate the fact that manga books are never available in my country, and if they are, it's in a minimal quantity. I wish they opened up one of these here...