Correct me I'm wrong, but either an immunization works, or it doesn't, right?
An immunization is a weakened or killed version of what you are trying to immunize against, to allow your body to produce the antibodies required to fight it off. Your body needs to learn the receptor points of whatever you are trying to prevent. With no immunization, it has to start from scratch and hope it figures it out before the illness can replicate to the point where it overcomes your system or the symptoms do. A weakened or killed version has the required receptors, but is not able to replicate fast enough, or at all, to allow your body the time it needs to develop its antibodies.
Once it figures out the receptor points, and knows how to make the antibodies, you basically become immune, or have less severe symptoms. In some cases, you apparently do need boosters, like tetanus shots, but I am not exactly sure why.