Sometimes I vote on an answer because at the time it is correct (upvote), or they do not explain very well and it looks wrong (downvote).
As time passes a new answer is posted, and in context you see you up/down vote was incorrect. Maybe the new answer shows you shouldn't have downvoted a badly explained answer or that as times change the previously correct answer is now incorrect. The old poster is no where to be seen, and isn't going to edit their answer any time soon.
If we can't have total freedom over recasting our votes, this would be a good tool to keep the site up to date.
FYI this feature-request would help solve this support question: Locked-in votes are unhelpful for old answers with new solutions.