The Biblical writers were not just plagiarists, by the way; if they took the story from the Babylonian Gilgamesh epic, they were doing so with a purpose, just as they were doing with the creation myths. The creation myths were deliberately meant to echo other creation myths of the time, but to impose a Jewish religious meaning on them. Thus, the writers of the Noah myth were taking an ancient story that had probably been retold time after time, and "sanctifying" by giving it the "right" interpretation. In a way, they were doing the same as many early Christian missionaries did in Britain: they "baptised" the old gods and made them saints. This meant they no longer had the power over people and things that they previously had (that power was reserved for God) but they still had importance as symbols.
It's also what you get in such Christianised epics as Beowulf, for instance: based on old Pagan stories, but given a Christian meaning. It was part of a propaganda war, an argument with paganism. None of it was ever intended to give any historical or scientific information. It was part of Judaism's "Our God is bigger than yours" campaign.