This could be important in determining the timeline of events; to see if the physical state of the drives match up with the data on the machines. Hard drives degrade over time naturally. They lose their charge or otherwise break down over time. I'm thinking if you looked really closely (like down to the molecular level) you could tell if the data was written on election night or if it was written over with falsified data.
You can not do this with an image copy of the original drives.
Technically, if you're looking for data, the original drives will be evidence. If you just dig through them you're altering the evidence and a defense attorney will have a field day. The correct way to do it is to make a forensic image of the drive and use that. This way your evidence doesn't get messed up (time stamps, etc). If it's imaged properly it is fine. I used to work with forensic computers.
An image can't copy magnetic hysteresis though, which is what I think OP is referring to.
In layman's terms, if you overwrite a hard drive's data in order to "erase" it, you can still measure traces of the old data. At the very least, you can show that the data was altered, but in some cases you can recover the old data.
What I'm saying though is the accepted method for examining a hard drive, forensically, is to create a forensic image of the drive and examine that. You never examine the actual drive directly as it alters the evidence. It's quite easy to obtain deleted data off of a forensic image of a drive. I've done it a lot. Maybe I'm not understanding what he was saying though.