Does locking your door really prevent break-ins?

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?) ... ^_^

Locking the door prevents walk ins.

Also, I think some home-insurance theft policies are invalidated if your doors are unlocked (during the breakin).

Yes it does help to prevent breaking and entry - u might also want to check that u dont leave any household work tools in the yard - i read somewhere that many burglars use them to help them break the locks etc

just don't make things easy for whoever's trying to steal from you ... if they're drunk (for example) and thus more likely to break the law, you want them to work that wobbly painful fuzzy head of theirs to break into your house ... hopefully, they'll think it's too much work and go away, in which case you would file an "attempt to breakin" instead of an actual "breakin" (yay!) ... btw, i'm sorry if it's painfully obvious if I've never drunk anything stronger than ginger ale ... ^^

If you were not locking door then in theory they wouldn't be breaking in, just entering.

Actually, the more locks you have locked on your door, the greater the chance that the burglar will pass to, let's say, your neighbor's door with fewer locks. Thieves hate the hassle like all people do. And they are really motivated to move fast. So...
A single lock prevents the thief from opening the door, grabbing the nearest bag, and running away, when you're at home. Useful.
Barred doors or cave entrances started as ways to protect home from wild animals.

@nemezide - yeah, that's probably true too ... if I was stealing from someone's house i'd be all nervous and now want to hang around to get caught by neighbors