It's obvious that you wouldn't just go into a car showroom and ask, "what's the best car?"
The salesman's reply would obviously be, "what do you want it for?"
(or he would just show you the highest-spec, top-of-the range model - taking "best" to mean the one that gives him the most comission!)

Similarly, you wouldn't just ask your greengrocer, "what's the best apple?" and not expect him to say, "what do you want it for?"