Deadbolts help keep petty criminals, or awful neighbors, out of your house while you're not home. All keyed deadbolts should be locked every time you leave the house.

 

Some folks think that it's better security to have double sided deadbolts (deadbolts that are keyed on both sides, so you need a key to unlock it even from the inside) on their doors to prevent someone from breaking the glass panel and reaching in to unlock the door. They might be right, but in my experience of seeing the aftermath of this situation, that will only slow them down and cause them to break more stuff to get in. Besides that, most of the time I see double sided deadbolts in peoples houses, they always leave the dang key in the lock on the inside anyways, defeating their own argument. Besides that, having a door that's locked from the inside that you need a key to get out is a major fire hazard. If you need to get out the door quick, you can't be looking around for your keys first!