The point is the same for the existence of any of the endless other fast food places out there. People love their garbage and being unhealthy is a way of life now.. it's not changing any time soon, so there will only be more and more of these places in the future. The point is sugar (and salt and fat etc etc from false sources).. if you can build a business around it, you will die filthy rich. Early if you eat your own food lol..
The point of Starbucks is to make money, and even further wave the prospect of deteriorated health under the noses of the masses, whilst the 'elite' laugh at all of the money that they rake in upon the back of that. One of my worst dates ever was in a Starbucks, so perhaps that is why I am not particularly fond of the place. But then again, the coffee sucks from there. It doesn't taste very good, and you have to add like ten thousand sachets of sugar before you can actually taste the sugar.
The point is that, no matter where in the world you are, if you go at a Starbucks, you know what to expect. You know it's quick, you know the product and the price for value you will get. It's convenient that way. I like the Starbucks smoothies. I don't drink coffee though.
Brand recognition and proper advertising. Sure, there are tons of places where the coffee is better but at the moment this company has a notoriety on it's area. It's a modern service and it was advertised in such way to make it more appealing for the customer. It can also serve as a social gathering spot same as every fast food restaurant in town. Hey, let's grab a quick Starbucks and sit down to catch up. The current generation embraced it as part of their culture so it's kinda hard to be against it.