I mean technically they spin to cool but also because they use the time it takes to rotate to unload and reload a round. Basically "1 fire, 1 move out of the way, 2 fire, 1 unload, 2 move, 3 fire, 1 load, 2 unload, 3 move, etc." multiple reasons

As someone else put it. No dakka, just brrt. When you have 8 dakkas in sync, you get all the -kka happening while the next barrel is already Da-, so dakka dakka turns into dadadadadadddddddrrrrrrtt