Icons on the minimap for rares and treasure chests, called "vignettes" by Blizzard, are a staple of the last few expansions. They lead us to shiny loot, achievement criteria, and so very many grey items.
Sometimes, we know about them through the API when they're not showing up on the minimap. So, show them as soon as we know about them rather than waiting for Blizzard to get around to it.
This is particularly useful if you're using, say, SilverDragon or NPCScan, which alert you about rares as soon as the not-yet-visible vignette is there, leaving you all confused as to where you're actually supposed to go. It also tells you about chests, though, so the HandyNotes crowd can be happy as well.
The icon shows in grayscale when it's displayed by this addon, and switches to color when the Blizzard display kicks in, so you can tell when other people would be able to see it. (On the theory that this is probably helpful when grouping.)