They are trying to avoid a situation where you end up with one juror who watches a lot of CSI and insists that they need forensic evidence to convict, despite having a dozen eye-witnesses. If a juror cannot imagine a circumstance where the evidence could be beyond a reasonable doubt based on non-forensic evidence, then they aren't suitable to be a juror.
Eye-witness evidence: so unreliable it's an active insult that people gesture towards it as one of the pinnacle forms of evidence.
Reminds me of the Norm Macdonald bit where he says he would not convict someone based on DNA evidence. The punch line is something like "I'm not a scientist, what do I know about DNA?"
For example, if you're sitting in your living room with a bunch of other people, many of whom know each other, and two people start fighting, you are all witnessing a crime and you can also all identify the two people fighting. It would be ridiculous to require DNA evidence in that situation.