It prevents the opportunist burglar from having your car away.
Obviously not the nasty fcukers who have targetted your car and WILL take it in any event, and using whatever means necessary.
Even if that means car jacking, which all of the alarms and guard dogs in the words wont protect you against.
So aside from carrying a gun, the harsh reality is that these poeple will take you car from you if they want it. Fact.
And until we have proper punishment and deterents in this country, this will never change, except to get worse.........but thats another debate and wont help the OP.