The idea behind locking doors is to make things slightly challenging for a casual burglar.

My parents live out in a rural area, and they never lock their doors, house or car. The odds of someone driving to their house and burglarizing it are just too low to worry about it - and if someone were specifically targeting their house, they could just break a window and get in that way.

In denser areas, however, that logic doesn't make sense; it's trivial to case dozens of houses in five minutes just by driving down a street.