Even a door equipped with a high-quality lockset is an easy target
I know from personal experience that burglars and vandals have a field day when they come across an exterior door unprotected by a dead bolt. Even a door equipped with a high-quality lockset is an easy target because the latch might not extend far enough into the door frame to withstand a sharp kick. A well-placed boot will tear the strike plate loose and splinter the jamb, providing easy access to a house. Installing a dead bolt in addition to the lockset you already have solves the problem.
When you buy a lock, don't look for bargains. There might not appear to be differences between a $6 dead bolt and a $16 model. But there are. Stick with a name brand. I've had good luck with Baldwin and Schlage hardware. If you're willing to spend an extra $10, you'll get a dead bolt three times as strong as the economy model. Whichever brand you choose, look for a hardened-steel bolt that extends at least an inch into the doorjamb.
For doors that contain glass or entries with sidelights, I stay away from locks with a thumb latch on the inside. A burglar can break the glass and unlock the door. For these cases, a dead bolt with a key inside and outside is my favorite, but this choice is controversial because it makes it harder to get the door open in a hurry. That is a concern in case of a fire, especially if you have young kids. In these cases, it's a good idea to keep a key near the door but out of sight and reach of someone on the outside. Some building codes require that at least one entry have a thumb-turn latch on the inside. Bottom line: Check your local building code before deciding on which type to install..
Even a top-quality dead bolt is only as good as the weak link in the chain, and that's the doorjamb. Some manufacturers reinforce the bolt with a steel pocket that is mounted in the jamb behind the strike plate. I like to go one step further. When we install dead bolts, we add an extra piece of steel strapping to make sure the bolt won't blow out the back of the jamb if it is kicked. We take off the door casing and install the strapping on the inside edge of the jamb, right behind the strike plate (see illustration).