One of the side effects of locking in someone's vote (after an initial five minute period you have to undo a mistake) is that it forces folks to be much more deliberate and thoughtful about what they vote for.
The purpose of voting is to help a community vet that information to help assure it is useful and correct. If you "vote first, read later", it has an unfortunate side effect of attracting more votes like it, whether they are ultimately warranted or not.
It sounds a bit odd, but there is an an unfortunate but very real social bias that people tend to join in and believe something to be correct simply because the group says so. That type of confirmation bias has been used to game the system where folks go around indiscriminately up-voting each other's post simply to give them that initial momentum to gain more up-votes early on — then they simply come back later to remove their vote so the suspicious activity goes undetected. The same can be said for tactical down-voting to push competing answers down the list so they can gain an early advantage. By "locking votes in", people are more accountable for what they vote for, and those who would otherwise engage in suspicious voting are more thoroughly routed by the system.
That's why votes are locked in after five minutes. You are ultimately responsible for what you vote for; so unless the information contained in the post is edited and changed, you should know what you are voting for before you click that button.