A good ground will help SOME, but not tremendously. It's partly for static discharge and partly to filter out ElectoMagnetic Interference (EMI).
Here's a little science lesson. Most folks believe that lightning travels from cloud to ground. That really isn't the case at all. Imagine this....You're holding two electrical lines of standard household 110 current. You slowly inch the two hot wires closer and closer together until eventually they arc, causing a spark. Lightning is the same thing only on a MUCH bigger scale. Static charges build up in the clouds, and also on the ground. As charged cloudds move over an area of charged ground....BBOOOOMMMMM. You get lightning. Wind blowing over antennas, dishes, builds up static electricity. Albeit a very small amount. If your antenna is grounded, then these static charges are constantly dissipated to ground, thereby incresing your protection.
Make sense?