this is just common sense answer ... but people probably first started locking doors for privacy reasons ... especially seeing as how people have more tendency to lock doors in cities than in country-side (may not apply at your location). locking doors doesn't prevent break-ins, they just discourage them. Imagine if you're a thief ... would you rather rob a place with the door already open, or a place where you're going to have to figure out how to pick the lock? Thieving is often "an act of opportunity" ... if you decrease that opportunity, then you're less likely to get stolen from (doesn't mean you won't, but less likely is better than nothing, right?) ... ^_^