The notion of a national ID number is being considered for a variety of reasons: the Social Security number was never intended as a national ID number, but is used that way, the illegal immigration crisis is due in part to the difficulty of identifying valid U.S. citizens, and law enforcement, insurance agencies, and health care providers all like the idea of having a better way to keep tabs on people.
Meanwhile, we have Google and the credit card companies tracking and aggregating information on everything we do.
For an idea of how this might all work in the future, check out this funny but frightening demonstration.