Traps, grip strength, core, legs, pretty much everything gets crushed. You can go for weights as heavy as you can carry for 10-20 yards, or grab some light kettlebells and try to take 500-1000 steps with them(try that shit sometime, i DARE you, lol).

Another thing about them that makes them incredibly valuable in this modern age of sitting, is that they pull your shoulders down and back, basically putting them in traction. This is amazing for shoulder health and for posture, and the cool thing about it, is that it just happens on it's own while you're doing the exercise, don't have to think about it, work on a fancy technique, etc.

push. pull. hinge. squat. carry.