Why would using an eraser holder be better than using a normal rectangle eraser? I saw this today:

It's a Staedtler Mars Plastic Eraser Refillable Holder.
Well, if you tell me that it can be used to clean corners, yes initially it would but soon its edges would become round like other erasers. How will that be helpful?
If you say that it can be cut to get a new sharp edge, well then it can be done to other rectangle erasers too.
When and why should this one be preferred over the rectangle erasers?
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.