If you have one "seamless skin" model (like a character that's one continuos mesh) you need bones to deform (animate) it - bit like your own skin and skeleton / muscles inside it. Bones themselves are not enough, you need a way to tell the bones what vertices to move when the bone moves, so you need weights. you can also assign same vertices to several bones so that you get smoothly moving joints, ie organic animation.
You CAN do "rigid body" animation with bones too, like robots or tanks, you just then assign the whole one mechanical piece (like an arm or turret) to one control bone only instead of doing the usual / old "hierarchial animation" way.
You CAN do "rigid body" animation with bones too, like robots or tanks, you just then assign the whole one mechanical piece (like an arm or turret) to one control bone only instead of doing the usual / old "hierarchial animation" way.