A. Two reasons. 1) The pre-sliced Challah is sliced thin, while I cut thicker slices – which makes for a tastier chunkier piece of Challah, plus it may be softer and fresher being that it is sliced right there on the spot. 2) Although all Challah is good, they may want to partake of the Challah upon which we made the blessing. It is also because we prefer to make the blessing while Challah is still whole as originally baked, before it’s cut.