resize_root=YES in rc.conf ????

[Ottavio Caruso <ottavio2006-netbsd%yahoo.com@localhost>]

From netbsd-announce:

On 18 August 2015 at 20:31, Soren Jacobsen <snj%netbsd.org@localhost> wrote:
> Some of the changes since 7.0_RC2 are:
> - Add a resize_root boot operation (disabled by default). If
>   resize_root=YES in rc.conf then the system attempts to resize the root
>   file system to fill its partition prior to mounting read-write.

Can anybody explain in layman's terms what that means and why I would
want to enable that option?

Thanks

--
Ottavio