I believe it was worldwide, and, YES, it's important.
Biblical Reasons
A few of the many Biblical reasons for believing in the global Flood are briefly summarized below. For those who believe in the Bible as the inerrant word of God, these should be sufficient.
Jesus Christ believed the Old Testament record of the worldwide Flood. Speaking of the antediluvian population, He said: "The flood came, and took them all away" (Matthew 24:39). Evolutionary anthropologists are all convinced that people had spread over the entire Earth by the time assigned to Noah in Biblical chronology, so an anthropologically universal Flood would clearly have required a geographically worldwide Flood.
The apostle Peter believed in a worldwide hydraulic cataclysm. "Whereby the world [Greek, kosmos] that then was, being overflowed [Greek, katakluzo] with water, perished" (II Peter 3:6). The "world" was defined in the previous verse as "the heavens . . . and the earth." Peter also said that "God . . . spared not the old world, but saved Noah . . . bringing in the flood [Greek, kataklusmos] upon the world of the ungodly" (II Peter 2:5). Note also that these words katakluzo and kataklusmos (from which we derive our English word "cataclysm") are applied exclusively in the New Testament to the great Flood of Noah's day.
The Old Testament record of the Flood, which both Christ and Peter accepted as real history, clearly teaches a global Flood. Therefore, it seems to us that Christians, professing to believe in Christ and follow Him, can do no less. For example, the record emphasizes that "all the high hills, that were under the whole heaven . . . and the mountains were covered" (Genesis 7:19,20) with the waters of the Flood. This must have included Mount Ararat on which Noah's Ark landed, and which is now 17,000 feet high. This was no local flood!
Since "all flesh died that moved upon the earth . . . all that was in the dry land" (Genesis 7:21,22), Noah and his sons had to build a huge Ark to preserve animal life for the post-diluvian world—an Ark that can easily be shown to have had more than ample capacity to carry at least two of every known species of land animal (marine animals were not involved, of course). Such an ark was absurdly unnecessary for anything but a global Flood.
God promised that never "shall there any more be a flood to destroy the earth" (Genesis 9:11), and He has kept His word for over four thousand years, if the Flood indeed was global. Those Christians who say it was a local flood, however, are in effect accusing God of lying, for there are many devastating local floods every year.
Scientific Reasons
The earth's surface and sedimentary crust also bear strong witness to the historicity of a worldwide Flood, and the early geologists (Steno, Woodward, etc.) taught this. Most modern geologists have argued, on the other hand, that the earth's crust was formed slowly over billions of years. Yes, but consider the following significant facts.
All the mountains of the world have been under water at some time or times in the past, as indicated by sedimentary rocks and marine fossils near their summits. Even most volcanic mountains with their pillow lavas seem largely to have been formed when under water.
Most of the earth's crust consists of sedimentary rocks (sandstones, shales, limestones, etc.). These were originally formed in almost all cases under water, usually by deposition after transportation by water from various sources.
The assigned "ages" of the sedimentary beds (which comprise the bulk of the "geologic column") have been deduced from their assemblages of fossils. Fossils, however, normally require very rapid burial and compaction to be preserved at all. Thus every sedimentary formation appears to have been formed rapidly—even catastrophically—and more and more present-day geologists are returning to this point of view.
Since there is known to be a global continuity of sedimentary formations in the geologic column (that is, there is no worldwide "unconformity," or time gap, between successive "ages"), and since each unit was formed rapidly, the entire geologic column seems to be the product of continuous rapid deposition of sediments, comprising in effect the geological record of a time when "the world that then was, being overflowed with water, perished."
It is also significant that the types of rocks, the vast extent of specific sedimentary rock formations, the minerals and metals, coal and oil found in rocks, the various types of structures (i.e., faults, folds, thrusts, etc.), sedimentary rocks grossly deformed while still soft from recent deposition, and numerous other features seem to occur indiscriminately throughout the various "ages" supposedly represented in the column. To all outward appearances, therefore, they were all formed in essentially the same brief time period.
The fossil sequences in the sedimentary rocks do not constitute a legitimate exception to this rule, for there is a flagrant circular reasoning process involved in using them to identify their supposed geologic age. That is, the fossils have been dated by the rocks where they are found, which in turn had been dated by their imbedded fossils with the sequences based on their relative assumed stages of evolution, which had ultimately been based on the ancient philosophy of the "great chain of being." Instead of representing the evolution of life over many ages, the fossils really speak of the destruction of life (remember that fossils are dead things, catastrophically buried for preservation) in one age, with their actual local "sequences" having been determined by the ecological communities in which they were living at the time of burial.
The fact that there are traditions of the great Flood found in hundreds of tribes in all parts of the world (all similar in one way or another to that in the Genesis record) is firm evidence that those tribes all originated from the one family preserved through the cataclysm.
http://www.icr.org/article/842/