Dave has pretty well defined the 'what' they are in your question. The why is simply this. The rifle is an inanimate object. For it to become a weapon requires the interface of a person. The main interface on any weapon is the trigger. Heavy triggers make accurate rifle shooting nearly impossible but light triggers with large amounts of creep and overtravel are nearly as impossible to master. An example - I have a CZ 452 that has to be the most accurate 22 rimfire I own, maybe one of the most accurate I've ever owned. I also have a 52 Winchester, one of the currently made variants with the 'micro-motion trigger'. The 52 while a really great shooting rifle is not a mechaically accurate as the CZ but the CZ's trigger can't hold a candle to the micro-motion trigger of the M52. In the woods hunting squirrels they are both great rifles but the CZ demands a bit more. The M52 is easier to hit with and that makes up (easily) for any difference in intrinsic accuracy.. Off a bench the CZ always wins, in the woods, the winchester is easily as good. Now if I could just figure a way to transplant that Winchester masterpiece into the CZ, I would have true greatness. Just as a note. Trigger pulls should be measured with the rifle muzzel down and solidly held. The guage is attached to the center of the trigger blade and pull stright up paralell to the centerline of the rifle and centered on a line through the center of the butt plate.
