As with any custom that has been around for well over two thousand years, there are many rituals and traditions. For example, at the Sabbath table, there are
usually two challot (plural of challah), to represent the manna in the wilderness that was doubled on the day before the Sabbath. There is also a custom not to
[T]he challah is kept covered before it is eaten, because … well, now we’re getting esoteric.
–From Dex’s mailbag response to a question about challah
Dex! You stopped in the middle of something interesting!
The reason challah is kept covered right up until motzi (the nickname for the blessing said over it) is that on the eve of Shabbat, a blessing is said over both wine (fruit-of-the-vine) and challah at the outset of the meal. The blessing over the wine is said first, and the challah is kept covered, so it will not know the wine blessing (kiddush) is being said, and therefore will not be embarrassed to come second.