What do "Collector" specialists do?

I've been playing Disgaea 2 a lot lately, and while grinding items through the Item World I encountered a few specialists named "Collector".

The wiki page on specialists does not, unfortunately, explain what a Collector does aside from "increase rarity" without explaining what rarity means in this context.

Can anyone here elucidate?

Each one increases the rarity of the item it is contained in by 1 (reduces the rarity number by 1). The rarity in this case is the number from 0-255 that determines if an item is common, rare, or legendary. The biggest use of these is to change the rarity of equipment with rarity numbers that are near each other in order to get them to match to give you a bonus to the stats granted by each item.