I have a US passport. I use it when I go to another country.
When I have traveling within the US I never take my passport. There is zero need and I may lose it so why even bother taking it when it serves no use?
My trip was a 6 week long road trip from the south eastern US through Texas, New Mexico, Arizona, California, up to Seattle in the northwest, and then back to the south eastern US.
I had no intention of going to Mexico or Canada. They are fine countries but that was no the intent of this trip so I left my passport at home.
During this trip I learned that the US border patrol has set up border inspection points about 10 to 50 miles inside the US. Most of the southern border are remote desert regions where people can cross on foot and then try to meet up with someone in a vehicle on the other side. So the border patrol doesn't just patrol the actual border but has checkpoints inside the US on the roads near the border.
You don't need a passport to get through these as you never left the country.
But I was driving a vehicle (Honda CR-V) with an open cargo area that showed it was full of luggage, bags of food, and lots of water bottles. I probably looked suspicious like I was picking up immigrants on foot or smuggling drugs.
I wasn't. I was on a big road trip and wanted to be prepared for days if my car broke down on a side road and I was stuck in a desert area waiting 3 days to be rescued.