They allow the security guard(s) to notice and catch pickpockets. They also prevent vanalism. In the zoo information, you can see how many instances of vandalism occurred, how many people were victims of pickpockets, how many pickpockets were chased and caught. If you don’t install security cameras to basically cover all areas with guests (particularly crowded areas), these numbers are high. With cameras, you get only a few or none (the actual numbers obviously depend on the total number of your guests). Guest really hate to see vandalized objects or get pickpocketed (does this verb exist?) and their happiness will suffer.
There is a heatmap for security coverage,