I know this is NRL, but I'll answer it with an overall perspective on collecting cards, rather than brands, etc.
For me, I love numbered cards. Whilst it's true that every card produced is a limited print (even 1,000,000 cards are limited print), I think the idea of showing the actual print run of a specific card is great. Sure, a lot of us over-look the numbering, etc, but in reality let's pick a card #/10. There can only be ten people in the world who can ever own that card at any one time (unless they move it on, I mean).
To me, that's an honor in some aspect, as being a very limited number, and I'm one of the people who can own that card.
Now let's take a 1/1, whilst the production of 1/1s these days is pretty high (in that every man and his dog have a hand full of them), you're still the ONLY person in the world with that specific card! It's pretty cool when you think about it (I believe).
As Adam mentioned, there's a lot of people who also put added value in the first card (01/25) and the last card (25/25) and also the player's particular jersey number (Jordan 23/100). I personally don't hold any more value in it, but I do appreciate the Jersey Numbered stuff.