when exactly are we supposed to use "public static final String"?

I have seen much code where people write public static final String mystring = ... and then just use a value.

Why do they have to do that? Why do they have to initialize the value as final prior to using it?

UPDATE

Ok, thanks all for all your answers, I understand the meaning of those key (public static final). What I dont understand is why people use that even if the constant will be used only in one place and only in the same class. why declaring it? why dont we just use the variable?