In some cases, achievement farming is necessary to get these new weapons. Sure the Medic achievements seem tame, but back when you had to get all 36 for the Ubersaw so you could charge your uber anytime you melee was insane, especially for achievement such as going on a 20-kill spree while healing a heavy, getting 1000000 heal points, blowing up 5 buildings with a demo's stickies in one uber, etc. For the pyro, wherein Valve got smart and made the achievements actually possible by the average player, a guy can unlock the Backburner and unleash his fiery fury with full pride knowing he did it legit. Plus he doesn't have to do some of the more ridiculous ones like 1000000 fire damage.
But you don't play the game. I wouldn't expect you to understand.