The Flood story in Genesis plays a central role in the arguments of Scientific Creationists — more central than many outsiders seem to realize. The Flood story isn't used by creationists as a means for simply trying to demonstrate that Creationism can be scientific; rather, it is also a means for trying to undermine evolution. The Flood story helps demonstrate the extent to which creationism ultimately depends and relies upon fundamentalist religion rather than science or reason.
As Henry M. Morris explains in his 1974 book Scientific Creationism:
The Genesis Flood is the real crux of the conflict between the evolutionist and creationist cosmologies. If the system of flood geology can be established on a sound scientific basis, and be effectively promoted and publicized, then the entire evolutionary cosmology, at least in its present neo-Darwinian form, will collapse. This, in turn, would mean that every anti-Christian system and movement (communism, racism, humanism, libertinism, behaviorism, and all the rest) would be deprived of their pseudo-intellectual foundation.
The person most responsible for the development of Flood Geology as a central doctrine in Scientific Creationism was George McCready Price, a Seventh-Day Adventist. Price's 1902 text Outlines of Modern Christianity and Modern Science was the first effort to harmonize a completely literal reading of Genesis with scientific investigation of geology and nature.
According to Price's analysis, geologists dated the age of rock layers based upon which fossils were present; however, those fossils were themselves dated based upon which rock layers they were found in. This is a circular process, Price argued, and so it was impossible to prove which fossil is older and which is younger.
During the early days of the 20th century when he was first promoting his ideas, Price found few converts outside of his church. As a matter of fact, for a long time Flood Geology was automatically discredited among many creationists precisely because of its origins in Seventh-Day Adventism — the Adventists were premillennialists who expected an immanent return of Jesus Christ and were dependent upon a very literalistic reading of the Bible in order to develop that doctrine.
This might not sound like a problem for creationists today, but in the late 19th and early 20th century most Christians were not premillennialists and did require the Bible to be read so literally. As a consequence, groups like the Adventists were considered theologically suspect. If these arguments really were "scientific," why were they rejected based on theological objections? Isn't it interesting that theology was taking precedence over science even in these early days?
Once Christians in America made their own theological shift to premillennialism, literal readings of the Bible became more mainstream and the Young Earth Creationism which depended upon it slowly gained ground. It would not be until the 1960s, though, when Morris and Whitcomb wrote their influential book Genesis Flood and founded the Creation Research Society did Price's ideas about geology become dominant among most fundamentalists.
Morris and Whitcomb were successful because their work provided a synthesis of the biblical text and scientific data which others had thus far not managed. They were "theologically correct" while also creating a veneer of rational, scientific credibility. Their work played a major role in helping cause the final shift of creationism in the United States from being predominantly Old Earth Creationism to the more extreme Young Earth Creationism.
Old Earth Creationism didn't depend very heavily on the Flood because so much of what we see around us could be accepted as a product of long geological changes. For Young Earth Creationists, however, there hasn't been enough time to great changes to occur. This problem is presumed "solved" if we assume the occurrence of a world-wide catastrophe like a Flood. This "explains" why fossils appear in rocks on mountain tops and why some animals are now extinct. Without the Flood story from Genesis, there's nothing for Young Earth Creationism to rely upon.