What do the ferrite cores for Nikon D3100 do?

Are they necessary to use?

Ferrite cores are used on cables that transfer data that could be corrupted if exposed to RF waves.

They may be overkill, but don't remove them.

If you are overly concerned, just use a card reader and be done with it. For nearly a decade, experienced digital shoots have been using USB 2.0 card readers with excellent success.