Normally, the Grounded Conductor [AKA the Neutral, or Noodle to be more specific
] would only be Ground Bonded at the main service [we will not include the PoCo's Grounding at the Transformer, since this will complicate things]. If there is a Transformer which the customer has connected on the load side of the main service, the noodle for that system will be grounded at that Xformer only [I'm keeping things simple here!]
Some times a noodle will be re-connected to a grounding electrode system for a sub panel [separate buildings]. This one needs to have only the Earth its self as any conductive path between points of Grounding [Grounding Electrode Systems, or connections between EGCs of the two different panels].
To sum it up, having the noodle ground bonded at the service panel and at a sub panel will cause the Equipment Grounding Conductor to carry some current during normal operation - more than it is supposed to be carrying. This becomes an issue.
If the bonding is done at one universal place / panel, this would be the design intention and be proper Grounding for the noodle.
The point is to not have a parallel ["Neutral"] current flowing in the EGC, or any "Large" amount of current flowing on the EGC during normal operation [Large being something over 1 Amp at the most].
I'm not sure if this explains it well enough for you, so feel free to fire some more Q's this way and most likely another member can pick up the subject and explain it much better to you.