Given the implied importance of tradition for a wedding day, there are a lot of things that people superstitiously stick to even given their apparent inconvenience. One of these traditions is that the bride and the groom should not see one another from midnight on the day of the wedding until they meet at the altar. The idea is that, when their eyes meet in the witness of the wedding arena, they should look upon one another as if they were discovering anew the person with whom they will spend the rest of their life.
This is not, strictly speaking, a required part of the wedding and there are certainly plenty of couples celebrating major wedding anniversaries in the present day having spent the night before the wedding together. However, it is still stuck to by many, and can lead to logistical somersaults being turned by the couple in order to avoid running in to one another. In practice, this tends to mean the groom spending the eve of the wedding at the home of his best man, prior to a dash to the altar when they traditionally oversleep (this part is definitely not compulsory).
A couple who have lived together for some time prior to the ceremony may feel one of two ways – that they have seen each other every morning for a while and aren’t about to fix what isn’t broken, or that there is no point in tempting fate by breaking with tradition. In any case, it is not about what happens before the wedding, but what happens after it in the marriage.