sub-panel ground/netural connections
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?