>Redundancy!
>
>ICU Unit
>ATM Machine
>PIN Number
>Tuna Fish (What other kind of tuna is there? Do we ever say I would like a Bacon Pig Hamburger Cow?)
>VIN Number
>and ...

I'll give you the ridiculousness of ICU Unit and ATM machine, since the acronyms are 'spelled out'.

Tuna Fish is part of the vernacular, I'm afraid, as is Cod Fish.

But PIN and VIN are generally said as a word, so I guess people want to distinguish between a pin (a piece of metal with a pointy end) and a PIN by putting the 'number' after it.