Why did O'Brien give Winston a copy of Goldstein's book? Why didn't he just capture him beforehand? Why did he allow things to play out the way they did?

I realize the importance of incorporating the book to provide context to the reader. I just don't understand how it makes sense in context with the story.

If the answer is that he wanted to build a mountain of evidence against Winston before arresting him, then that's hypocritical of the entire last segment of the novel. The thought police don't have to have any evidence, they can do what they want. O'Brien could have simply told Winston that he read the book and it would have been true.

It's almost as if the book's presence in the novel is meaningless aside from providing the reader w/ historical context.