If my sub-panel is an extension of my main panel due to insufficient breaker space, then why is it necessary to separate my ground and neutral connections in my sub-panel?
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The electrical code requires it 2014 NEC 250.32. – Ed Beal Oct 20 '16 at 18:50
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Safety.
First of all, you'll get neutral current on the ground wire back to the main panel, because there are two parallel paths back.
Secondly, if the neutral ever fails, instead of just having no power everything will appear to be working, but you'll have all the current on the ground. This means now you can be electrocuted by touching the panel itself.
Edit: See my other similar answer: Why does a subpanel need separate ground and neutral?
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Neutral and ground are not redundant. They are completely separate systems doing completely separate jobs.
The neutral's job is to be the normal current return path.
The safety ground's job is to be a "shield" to protect humans from electric current, and shut down circuits where "hot" wires fault to ground, by assuring enough current will flow to trip the breaker.
To enhance safety, neutral is "pegged" to ground, to assure neutral's voltage is near ground's. This is done in one carefully chosen location most likely to have its safety features work properly. That is not some genius idea I could share, but rather, trial and error from studying incident reports.
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