I think one of the possible reasons why people buy coffee in starbucks is because we are familiar with it. When you go running to class or to work and you want a coffee, the easiest thing is to buy a starbucks because you know that, wherever you buy it, it will be exactly the same. however, when you evaluate the possibility of buying in another cafeteria, you may not like the coffee and maybe you do not want to assume that “risk”
another reason is its availability. Once you know what starbucks coffee you like, you know that you will find it all over the city and even in several countries of the world and you know it will be the same.
I think the idea of buying in starbucks to show off, depends on each person.