This question has been asked by so many people over such a long time. Why on earth do we have to bond the system neutral and the equipment grounding conductors, and grounding electrode conductors in an electrical system?
Well, it really comes down to “Because the National Electrical Code Says So” but there is a reason behind all of it. We didn’t have equipment grounding conductors back in the day, it was simply neutral and hot. As people began getting shocked when touching pieces of metal we devised a way to link all metal things around electrical equipment, together, and make them one continuous piece of metal.
Circuit breakers are designed to shut off power when electrical current goes through it at a high enough level. Allowing current to flow on these metal parts is extremely dangerous, so we need a way to make the current shut off when this happens. By bonding the equipment grounding conductors to the system neutral, there is now a path back to the transformer from where the power came. This allows for a full circuit to be formed, which will allow current to flow through the breaker in a complete circle from source, back to source, rather than just from source, to earth (where the breaker will not trip).
Bonding and Grounding are very complex subjects, which is why Article 250 of the National Electrical Code is one of the biggest sections of the codebook. There’s so much to know, and it all deals with saving lives, preventing injuries, and protecting equipment from damage.