Why do people choose Best Answer to questions that they have already decided the answer to prior to asking?

They just want confirmation of what they believe. This is human nature, just as it is human nature to be bothered by it. We all want to know that others think the way we do and having our beliefs confirmed by others in turn confirms our existence. It's still a pain in the azz, though, isn't it? It mostly stems from insecurity. The well-rounded individual does not need validation to feel good about themselves, but those individuals are few and far between.