A deadbolt is a type of lock that cannot be jimmied open once it has been turned, so they are often recommended by locksmiths as a way to improve home security. However, there are different types of deadbolts, and one of the most common distinctions is between single-cylinder and double-cylinder versions.
With a single-cylinder deadlock, a key is only required to open the lock from the outside, with a simple thumb-turn usually positioned on the interior side of the door. With a double-cylinder lock, a key needs to be used to open the door from both sides. Many people prefer single-cylinder deadlocks since they are cheaper and a little more convenient; however, they aren't quite as secure.
Reaching through to the interior
The fact is, there are a few easy ways for burglars to get a hand inside your home and open the door from the inside. Probably the easiest option is simply breaking a pane of glass next to the door, then reaching through and turning the thumb-turn. Another trickier but still possible solution is to use an improvised bent bar to reach through the letterbox; that bar can then be rotated until it hits the thumb-turn, which can then be quite easily rotated itself when the bar is handled by someone with a little experience.
The risk of bumping
One important characteristic of a double-cylinder door lock is that you can prevent someone else using a key from the outside simpler by placing your own key on the inside. If this doesn't sound very advantageous to you, it's probably because you haven't heard of lock bumping. This practice involves inserting a special 'bump key' into a lock, then gently bumping it; if the key is turned at the right moment, the lock will open. Bump keys can be ordered easily online, so this method is on the rise. You can prevent your locks being bumped when you're inside the home by simply slipping your key into the interior lock.
Building the habit
Ultimately, you should always lock your door from the inside, but it can be quite hard to keep to the habit when you only have a thumb-turn. In contrast, having dedicated locks on either side of the door can make that habit a lot easier to commit to. Just as you get into the habit of locking your door from the outside when you leave, you'll get into the habit of locking the door from the inside when enter.