Why would using an eraser holder be better than using a normal rectangle eraser?
I can think of four benefits of using an eraser holder:

It keeps the eraser clean so it doesn't pick up body oils from handling or other contaminants.  The eraser can be extended and retracted as needed, and you handle the holder rather than the eraser.  It minimizes transferring dirt to the paper and potentially smearing the graphite.
The holder provides some rigidity for the relatively soft, rubbery eraser.
The holder has the feel and control of a pencil.
It is a convenient form factor to carry around.  It's the size of a mechanical pencil or pen and has a pocket clip.