Mainly it's for the bank's protection, in case someone tries to duplicate the card. Since it's very hard to replicate a hologram, there's less of a chance for forgery. A hologram is essentially a three-dimesional photograph. However, it takes much more work to make a hologram than it does for an ordinary two-dimensional photograph. To learn how holograms are made, we first need to learn a little about lasers .
Interference is what holograms are all about.
Let's say we had a coffee cup and we shone some laser light on it. However, we didn't shine all of the light on the cup. So we have light straight from the laser source and light that's been reflected from the cup. These two beams interfere with each other, causing a specific pattern of bright and dark spots where there has been constructive and destructive interference, spots where the waves added and subtracted. Also, unlike the photograph, each point of the hologram contains all of the information from the object.