Exactly how do you convey a capital letter while talking?
Context. If I am ordering it at a restaurant, it's understood that I want a Coke, because nobody would order an unspecified "soft drink" (unless it's the sort of place where you fill your own cup, in which case that essentially is what you're ordering).
On the other hand, if I ask my wife to pick up some coke at the store, she knows I may mean Caffeine Free Dr. Pepper, Sprite or whatever else we need.