The Lake Pontchartrain Causeway is one of a whole lot of bridges in Louisiana and is the longest of all of them. Some other ones are the Bonnet Carre Spillway and the Twinspans, both of which carry I-10. They also run over the lake to the west and east of New Orleans, respectively. Those are the major three ways out of the Greater New Orleans area by crossing the lake, though US 11 does cross the lake, and US 90 comes close. I also believe the Causeway is the only major route out of New Orleans that doesn't have a number - at least that I know of.
Like some people posted above, it's the primary route for those living on the "Northshore" in St. Tammany Parish, and can also be used in the event of a hurricane evacuation.