Main purpose: deciding what armor to purchase is a very complicated and expensive process. The only way to preview right now is to visit the wiki galleries, but they are 2D pics in a single color, which don't do the armors justice. And you cannot mix and match pieces from different sets. As for crafting all possible combinations in-game, painting them and trying them out... that's quite an expensive endeavour. Sure, for some armors you can practice with lower AL pieces, but for higher-end armor that's not possible.
Features:
* Only pieces of armor for the character's primary profession, of course.
* You can mix pieces from different sets.
* [Optional] A way to apply dye combinations on the pieces of armor inside the previewer, without actually having those dyes.
* [Optional] Also make available other pieces of armor, such as glasses, masks etc.
* [Optional] Also make available costumes/festival hats.
Basically, a sort of "dress-up" mini-game with armor pieces for your profession.
PS: I would gladly pay 8.99 EUR for access to this NPC (one time purchase, full access for all chars on the account). Take my money, ANet!

PS2: I know that something similar was proposed as the fitting room. I think my idea is simpler and more straightforward. And since the original idea is from 2005, I think it needs a bump!