In stories, the background to the characters and events is almost a character in its own right. Creating the setting as I would create a character, so that I know its internal and external influences, its history and what it looks like, can help me figure out how the world around them shapes and influences the characters.
Generally I know far more about the setting than I ever need to use in any given story, more even than I would need for an entire series of stories, but there are still questions that I don't know the answers to. Sometimes these are answers that none of the characters would have either, sometimes it's just that the characters have no reason to ask or answer that question within the confines of the story.
A lot of these unanswered questions concern quantities: how many people share a particular belief? how many planets have the same kind of government as the one shown in this part of the story? how many companies provide the product or service that the characters require? These are all 'beans in a jar' questions. We know the approximate size of the jar, and the approximate size of each bean, but the exact number is only important if we want to win the jar of beans, or if we want to divide its contents equally between a set number of people.
Generally I feel that world-building needs to be detailed enough that a story has internal consistency and provides the reader with enough of a framework to understand how things work, but not so intense that the reader gets weighed down with the details or has no space in which to stretch their own imagination.
How detailed do you like your world-building? Who do you think is particularly effective at conveying the feel of the worlds they write about?