Imagine you meet a very beautiful girl and want to invite her to a restaurant. Unfortunately, she's not yet sure whether she'll be able to join you, but she is very encouraged and all. You decide to book a table in the restaurant, but it turns out that the restaurant is not engaged in free reservations and you need to pay a small amount for it. You decide to pay for the reservation, but you don't know whether you will use this opportunity or not. This way, you pay a small amount to have a 100% chance to visit this restaraunt. Options trading is absolutely same.