Bolts are a way of locking a door more securely, usually from the inside, giving the property additional protection. It might give you peace of mind, or it might perhaps allow you to feel more secure if you know your area is at high risk of break ins and burglaries.
A bolt works by providing an additional layer of protection. It means that someone cannot pick the lock, as there will still be a level of locking inside the door. This can make it more time consuming for someone to break in, so this may deter them. The trouble is that they will already have started trying to break and enter when they discover this, so it might be too late. Bolts, unfortunately, do not offer a visual deterrent in the same way as an alarm system, a strong door or security shutters might. However, they can certainly be worth installing for added protection.