>>63798950
Different mission sets and personal preference. Belt kits allow you to combine your first (immediate gear), second (fighting gear), and third (sustainment) line with a buttpack. You can carry a very large number of magazines with belt kits.
If you use your chest rig with a battle belt, you have two separate lines and can take your fighting gear off without removing your EDC / basic medical / hydration if that's how you have your belt set up. That makes it more complicated but also scalable.
Chest rigs are lower drag, higher speed, lower duration. Add belt + backpack and you can get duration back at the expensive of speed and drag.
Belt kits are higher drag, lower speed, higher duration. Easier all-in-one solution however..