Grip, traps, and core forged by the carry.
Stay tall and walk smooth. Don't lean or let the dumbbells swing into your stride.
The farmer walk is loaded standing still and then made brutal by movement, the traps and grip fight to keep two heavy dumbbells from slipping while the core and back lock the torso rigid against every step. Nothing is isolated, everything holds. It earns its place because carrying heavy things builds trap size, crush grip and bracing endurance in one drill, and the strength transfers to every deadlift, row and shrug you do.
Heavy enough that the last stretch of the walk is a genuine grip fight, light enough that you stay tall and never lean or stagger. A rough starting point is a pair of dumbbells that together approach your bodyweight, then build from there.
Traps and grip take the biggest hit, with the core and upper back working hard to keep you upright and the shoulders packed. It is one of the few moves that builds size and work capacity at the same time.
Completely, grip is the limiting factor for almost everyone. Keep training them and it catches up fast. Use chalk if your hands sweat, but skip straps, the grip work is half the point.
