You need to know what you are buying because usually you will need to cancel way after 24-hours.
Of course you need to know what you're buying, no one was saying otherwise. what Nitantnel was saying was that since you have 24 to cancel, so why do you need to know what you're buying BEFORE buying, why can't you find out afterwards. But i don't think that would make too much sense, because why would want to find out after you bought it that you bought the wrong thing and need to cancel right away, if you can find out BEFORE you buy it, and buy the right thing?