Basically just the title, just bought it and think I wasted some coins; it didn't seem to do anything?
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It's meant to take pre-dlc guns from max 300 attack to max 331. It can also be used to re-roll armor for better stats, *if* Xur happens to offer an upgrade better than the one you have now, even at the same light level. So even if he's selling a crappy version of something it's possible to buy it now and upgrade later, since the odds of upgrades showing up are higher than for sale. It's expensive, but can really be worth it.