It seems to have originated from this set of requirements and the ensuing discussion there. It seems to be kept for audit reasons (and, actually, that suggests that the date issue is really a simple bug).
I’m pretty agnostic as to whether we should keep this around for audit history (it seems to me that the snapshot of the condition list at the time a diagnosis or order is made may have some legal relevance), but if we do, we should probably have a previous_version pointer as we do for Obs.