If you want to throw a birthday party and invite all your friends, which of the following can be useful data?
Number of friends
Estimated amount to be spent on each friend.
Last name, height and weight of your friends are of course not relevant for planning the party. But according to the number of friends, you will get snacks, arrange for space etc. Also, if you know the expected amount to be spent on each friend, you can ask for sufficient money from your dad.