Dirt is a terrible conductor. That's why we don't make wires out of it.
Why it's called neutral: Neutral is supposed to be near earth potential - if you are standing barefoot in the rain and touch the neutral wire, not much should happen. The secondary purpose is to minimize the voltage between other conductors and earth - that's why neutral is "in the middle" rather than at a corner.
The jobs of earthing
Current travels in loops. Earth wiring has an important job: returning natural electricity to the soil: lightning and ESD. That's why the earthing spike (ground rod) is tied to the earth wiring.
Remember how we aim to hold the neutral wire at earth potential? That doesn't happen by wishing. There needs to be a bond between neutral and the earthing spike to establish that reference. It could hypothetically be any kind of bias, but 0V is easy to do with a piece of copper. This bonding could be any of several places, but obviously, it must be somewhere both neutral and earth wires are present.
And we need to return human-made electricity to source. If artificial electricity has a ground fault, where leakage occurs to the earthing system, we want that leakage to efficiently return back to source. Source is the supply (hot and neutral) - we're not so worried about neutral-earth faults, so we're mostly worried about hot-earth faults and we'll want to return that current to neutral. If it is a bolted fault (dead short), we will want it to trip the overcurrent device (breaker) so it must carry current. This requires very low-resistance -- dirt will not suffice. So it's back to a copper neutral-earth bond.
The grand union
These requirements, together, call for a grand union of the neutral wire, the earthing system, and the earthing spike. Long experience in North America has shown you want that neutral-earth bond in exactly one place -- and that should also be the place the earthing spike/electrode ties in.
If you don't have that grand union, you will be omitting some of the above-mentioned protections.
SOP in North America is for that grand union to be in the main service point (main panel), and neutral and earth to be rigidly separated in each subpanel.