1) Do they only cater after crafters/gatherers?
They can sell any tradable item. i'm not sure what you mean by "cater" though. You don't need crafters or gatherers to get items for them to sell, it just comes out of your (or their) inventory.


2) What’s the point of sending them for hunts, field explore & other once retainer is at 80?
Hunts can give you specific monster drops that might sell well or you might need for a recipe, Field explorations and quick explorations give you random drops that might either be unobtainable other ways (certain pets and gear) or untradable. XP for the retainer is one aspect but also as important is the items they bring back.



3) As a non-crafter/gatherer myself do I really need 2 more retainers?

It's extremely subjective. If we're speaking strictly do you NEED them to enjoy the MSQ and be able to hold enough storage to play the game, the answer is no. Between your armory chest, saddlebag, 2 free retainer storage and personal inventory you should have enough storage to keep gear and consumables for all your jobs. If you're going to dabble in Glamour collecting though space will become a premium. 400 slots in the glamour dresser seems like a lot .. until it's not.

4) Once retainer is at 80, does it make any difference on gearing them with 520 Cryptogear & increase the chances to get good rare items & other stuff?
There are thresholds for getting rare stuff but according to most research it doesn't matter how much you are over, just that you get the message that they can bring back rare items.