>>536598318
Weight painting determines how much each animation rig bone will affect the various vertices of the model when animating, it's how you decide what each bone will deform when you start animating

For example in pic related the red parts are the heavy weights that show which part of the arm will be most affected by the bone, the green parts are light weights that will only slightly deform the model, the blue parts will not be deformed at all

If the red weights were on the shoulder as well then that part of the torso would also be deformed when you move the upper arm which you don't want, with separate model parts this is not something you have to think about since every separate body part will just be fully red and modular and won't affect the others ones