That customer is likely to purchase coffee or something at Starbucks. Half a day is quite some time to go without having something to drink or having a bite to eat.

Presumably Starbucks has calculated this. There's new one near me on 53 and 3rd that has plenty of sitting space, and the food is arranged differently (more like Pret I guess). And there's a sign posted outside that anyone can come and sit without having to make a purchase. The coffee shop is, for many people - especially those starting a company or working on their own - their office.