Two example planes at 90t GW.

wing  fuselage  wing
  25t     40t    25t   (weight acting downwards)
    v       v       v

---o---+++---o---

  45t      0t       45t  (lift acting upwards)
    ^                ^

When you alter the fuel distribution while keeping the same GW, it may look like this.

wing  fuselage  wing
  10t     70t    10t   (weight acting downwards)
    v       v       v

---o---+++---o---

  45t      0t       45t  (lift acting upwards)
    ^                ^

Both planes are coming in at 90t but their structural loads differ a lot when it comes to the wing bending moment and the shear being caused.