Most Christians barely understood the ceremony and why of baptism when it happened to them.
It is an incompetent church or minister that would baptize anyone who has no idea at all of the reason for it. I would have to assume that's very rare. But if you mean "most Christians" don't understand it at the time as well they will come to understand it later with more maturity and experience-- there's nothing wrong with that. Certainly that was the case with the early Christians, but prompt obedience to the command to repent and be baptized were obviously high importance. And John Mark and Apollos, and most of the Corinthians had much yet to learn [or 'straighten out'], but there was no command for them to be rebaptized.
A big part of all this comes down to so much emotionalism in the youth camps and crusades in Vacation Bible School in which so many 'decisions' are made, then the same kids that made the decisions grow up a little and then many do want to do it over again, simply because they have learned a little more. The irony of these type of meetings is that they are pressed to make their decisions for Christ with the assurance of his being lord and savior 'for life;' but for other who have already made such a deicision, at the same meetings it seems they try to instill doubt ["You may have gone forward and got baptized, but you really know all it did was get you wet!..."] To the former group, the message comes out 'get saved and don't doubt it,' while for the latter it's 'doubt it and get saved.'