This a Type-Specifier function. It takes a date parameter and simply returns it. Its primary use is when you want to ensure that a data reference or prompt value is treated as being of type date in contexts where its role might otherwise be ambiguous.
Note: In version 3.1 there was a Date function which took 3 number parameters, specifying the year, month and day of a date, and returned the date. This function is now the DateAt function. The old usage is still supported for backwards-compatibility only, but should not be used in future - use DateAt instead.