Gaming

It duplicates an item exactly. PoE was all about RNGesus. I can't remember the numbers off the top of my head (and it got way more complicated over the game's life), but each rare (yellow colored) item can have 6 magical effects, 3 prefixes and 3 suffixes. Each of those effects can have a tier from 1-11 (different for some effects), and each of those tiers has its own range of numbers. So a truly amazing item has a super, super low chance to spawn (because getting 6 of the effects [and the right effects at that] to be tier 1 is hard enough, and then getting the max number within that tier takes even more sacrifices to rngesus).

Just as an example, let's say you wanted the 'super super best' armor you could get. First, you'd have to be playing at a really high monster level so that items have a chance to spawn the tier 1 effect (monster level=item level when dropped, and you need minimum item levels for high tier drops). Then you'd have to get lucky and have the right chest armor drop (because even at high monster levels, you can get the worst of the worst armors. So give or take a 1 in whatever chance for the armor, then a 1 in 20 chance for the right armor... Then you'd have to get the right effects, so that's a whatever in whatever combination calculation (I can't be arsed for that math, but let's just assume it's not too bad, so like a 1 in 300 chance), then each of those effects you'll want to be tier 1, which means for each effect there's between a 1 in 3 chance to a 1 in 11 chance (some effects only have a few tiers) to get that, then you'll want each of those effects to be at the max number for that tier, so grab your ankles and prepare for even more chances with a wide range...

See why I don't want to do the math? To get really, really good items is a really, really low chance. That means that if you have a really good item, it's going to be wanted by everyone else. Cue the dilemma: if you sell it, you'll make money, but you only have the one opportunity to sell it. If you could somehow duplicate it... well, sweet money, baby! So a mirror let's you duplicate an amazing item without losing the original. There was a famous dagger with spell modifications (most items are either melee based or magic based, but a dagger can have effects that boost either, which means you have even less chance of getting what effects you want) that a fella would charge beaucoups of money for, after you already had obtained a mirror of your own, and he would get the money, because the dagger was just that rare and valuable. Probably something like 1 in a billion chance of something that good dropping, so it was much easier to save money and trade for the duplicated dagger.

Oh, and an item that was a duplicate couldn't be duplicated again (it was 'mirrored'), only the original item could be duplicated.