You never know when you might be asked for it, or it might otherwise be needed.
Just because you don't have to carry it with you doesn't mean you shouldn't.
Next week the U.S. Supreme Court will hear a case to decide whether or not all Americans must have identification on them at all times. The case has been brought by a cowboy in Nevada who was asked to show ID while he was leaning against his pickup truck on the side of the road near his ranch. The police officer did not offer any specific reason why he demanded proof of identity. Having committed no crime, Dudley Hiibel, the cowboy, refused and was arrested. He was later convicted for "Delaying a Peace Officer."
An opinion, and links to the story are on The Argus.