Yes, people change their minds or make mistakes, and this is why we should always be able to change votes. Locking votes prevents this, and leaves good answers with downvotes they don't deserve, hurting the answerer and the readers.
Unfortunately, some people are dicks and give people needless downvotes because they're having a bad day, or because they want to promote their own answer even though the other answers are correct. This also leaves good answers with downvotes they don't deserve, hurting the answerers and the readers. This doesn't happen very often, because most people are decent, but it does happen sometimes and should be strongly discouraged. Locking votes does little to discourage this behavior. People can still downvote competing answers to make their own look good, and gain more points in upvotes than they lost in making downvotes. Locking votes doesn't prevent them from doing wrong; it just prevents them from undoing the damage later. The net effect is worse than if votes were not locked.
The solution is not to lock votes, but to incur a rep penalty for bad behavior. When you downvote a competing answer and then change it back to an upvote later, you should permanently lose rep, enough that there's no incentive to maliciously downvote others.