@jetah because I need 50 minna bird feathers, My skills are 90% ground-based attacks, so I have trouble killing the flighty jerks, and, besides, I need to go mine for the other ingredients for this Quetzal armor, and I don't want to waste time shouting "I need someone to kill a lot of Boids! anyone want to do this?!" for 5 hours and being entirely ignored. So I'm willing to pay some rando to go do this thing for me, okay?!
XD
Or maybe the town needs 500 Wolf pelts to upgrade a curtain wall, and no one wants to hunt all 500, so you set up a quest, put in the total reward in escrow, and anyone who wants to off-load a pelt can through the interface and get the reward without having to drag the mayor/merchant/town rep out to pay for it, assuming they are even online atm.
Or you're a merchant character, and you need some bodyguards because the eclipse is coming, and you've got a warehouse on Syn, and losing all your goods to a demon raid cause you were in the wrong place at the wrong time sucks.
Or your guild is going to siege a town, and you need to keep the allied town busy, so you quest up a merc raid to hold the second town at bay while you go murder hobo the first.
And, most pressingly, there aren't many NPCs (at least, not until a town attracts them), so being able to hand each other quests fills a need and acts as a general lubricant for the economic engine.