You have a simple question--you just want a book about football. So why is the librarian suddenly giving you the third degree about history of sports or rules or players or if it's for yourself or a child...?
I promise, it's not because she's trying to discourage you from a football book, or has an unhealthy interest in your motives. She just wants to get you the football book you really want, and the questions she's asking are called a reference interview. First, she needs to establish whether you mean American football or soccer, which is called football in most other countries. Then, well... check out the first of four pages of subjects relating to football. Do you want to know about the history of the sport? Fans? Particular teams? Coaches? Are you looking for the life story of a favorite player? Or just any player? Is it for your own recreational reading, or is it for a school report that has certain requirements? Are you just looking for a novel that happens to be about football?
But couldn't she just direct you to the football section?
Well, sure--it's around 796.332. You're welcome to browse it. You'll miss the biographies and fiction, but if you scan the shelf long enough, you'll find a few books on the sport's history, some books on rules, maybe some information on coaching. If you're just looking for something to read, that's a pretty good tactic. If, on the other hand, you need something particular--or if what you really want is fiction about football--then it's not going to work. You'll also miss a lot of books that might be exactly what you want if they happen to not be on the shelf at your branch at the moment you're looking! The librarian may also be able to help you find articles and other kinds of information that you might find useful.
The basic reason for the reference interview is clarification--to make sure that the person helping you is on the same page you are... and for that, it's good not to make assumptions. I remember one case at a previous job where a question seemed very simple. A patron just wanted books on wheels. A little unusual, but just a question of going to simple machines. But she was very frustrated, and went elsewhere. It was very clear to the next person that she was asking about whales. Even more frustrated, she finally established that she was looking for information about wills--a situation that would have been entirely avoided by properly asking what it was she needed to know about wheels or whales (or Wales, I suppose). A few simple questions from us can avoid a lot of frustration for you.
And that's why librarians are so nosy!