If there is a ground at the transformer which grounds the main entrance cables then why is there the necessity of a separate grounding rod at the meter or breaker box?

They serve two different purposes. The Neutral or "Grounded Conductor" provides the return path for the circuit from the pole (transformer and back.

At the main panel, both the Neutral and Ground are tied together also. But the use is totally different.

The ground in a main panel is used to carry large spikes (lightning) safely away from the residence and discharge it into the earth. Also, the ground in the main panel is used to carry "Fault" level electricity at a high rate to earth ground. This allows the OCPD (breaker or fuse) to open and kill the circuit thereby preventing fire or electrocution.

You ground the Neutral and main panel together to make sure that there is no "electrical" difference between them. If there was a difference, s small back channel could flow which would effect the operation of the ground circuit.

Thanks