You can answer immediately, you just can't accept it. Accepting is not very important. It doesn't indicate much, only that the OP considered this the best answer. As far as the system is concerned, questions are counted as answered if they have at least one answer with a positive score.
As for why the wait, it is probably just to let others also answer. The accept mark makes no difference to the system, but human psychology is another thing. Often, seeing an accepted answer will deter others from posting their own. Making you wait might just be a mechanism to allow the community to also contribute to your question.