The Zombie Stack Exchanges That Just Won't Die
Necessary in the sense of potentially needing to extract additional information from the physical object.
Bill Lefurgy
To correctly preserve digital media would mean having an exact binary image of said media, which by definition contains all the data stored on it. As a bonus, we could save a photo of the media in addition to the binary image.
After a proper binary image has been saved, the only reason to keep the original media would be for historical or nostalgia purposes, since the media would never need to be accessed again.
For example, I still have an original Zork I floppy disk for Commodore, even though I have an image of it that I use with an emulator. I'll be sad if I ever lose the original disk, but I'll always have the disk in virtual form, and I'll never need to access the original disk again.
Generally I would argue that it's the data that is important, not the medium on which it's stored. As long as the data makes it onto the most modern medium, the old medium can be discarded.
If you've extracted absolutely all of the content from the original media, including any information printed on the outside and perhaps also on the case, and you can confirm this has been done, then in most instances it should not be necessary to save the original media. But there absolutely must be confirmation that all information and content has been correctly copied before the original media is discarded. There should also be a record of the copying process - who copied, how, when, etc.
That said, there are inevitably instances where it is preferable to retain it, eg:
The original media should never simply be disposed of as a matter of course. Institutions should preferably have a policy stating and justifying the process before any destruction takes place.
I think this would depend on the kind of media you are working with. VHS tapes are very different "media" than CDs. Personally, I would be cautious about destroying any media, even if I believed I had extracted all of the content as faithfully as was present in the original.