Don’t you know that its really cool to have your name written on your car? Except you can’t actually write it or have it printed as that’s too easy and doesn’t cost much. You have to have it spelt wrong and with numbers like some sort of “leet speak” on the number plate. Better still, if you own an expensive car, you can get the name of the car, so that people don’t have to know about cars to know you have an expensive one, and they realise you are even more wealthy because you have spent thousands on a number plate.
On a more serious note, aside from the cynical reasons above, I’ve heard its about making a car unique and identifiable to you by other people. The different types you mention is difficult to do as there are fairly strict rules regarding what size, colour and font you are allowed.