When I was a kid, my dad had a business next door to our small, rural home. Although he always said, "A lock will only keep an honest man out," we did lock our doors at night.
When I lived in a nearby town, someone broke into the house while we were at the movies and took some valuable electronic equipment. And, the doors were locked, proving my dad's theory.
I now live alone in the country and had a situation where someone I knew -- who was struggling with some emotional problems -- would show up in the middle of the night, enter the garage, walk up to the back door.
I got into the habit of ALWAYS locking up everything, Even if I was only going outdoors for a few minutes, I locked the house behind me and carried the key. That situation is now resolved, but I still lock everything because it-- irrationally, I guess, makes me feel safer.