To guard and, equally importantly, to show in the code that this is an entity which is defined, but not going to be changed. Maybe it is because I always feel a bit uncomfortable with the idea of a variable which changes the value over the time. I feel more at home in the realm of functional programming, where you don't have assignment statements which modifiy the content of a variable.

Of course this is personal taste, but I still find it good practice to make clear in the code when an entity is supposed to be a constant.