Creases in trousers were introduced in order to emphasize the line of the trouser. Edwardian gentelmen were so used to the cut of narrow trousers that they were content to have either no crease or a crease on the side. Front creases only came into their own prominently when trousers were wide in the '20s and '30s. We have kept the creases in because they still look smart. But it should be noted that in jeans a centre crease is not only impossible but undesirable.