Do you mean it's still working with the wire cut, or am I misunderstanding you? As you probably realise, that white wire is an earth connection and it looks to me like it's been added to cure a problem that the previous owner may have been having with the dimmer.
Another possibility is that he/she might have been using the plug on the back of the dimmer to steal power for another device or light and needed a decent earth...I've done that in the past.
Interested in your reply.