I asked one year when casting my vote, why our card numbers were recorded against our names, my concern was that it removed anonymity.
I was told that they keep the papers for x number of years(I forget how long), locked away, just in case there is an issue around irregularities. They can then go back and check.
I've never followed up on this as the choice is simply vote or don't vote and I am unlikely to change the system, so this could be a load of rubbish. Interesting in this case if true though