In the text mode help:
   Selecting  Grow to fill disk allows that partition
   to increase or decrease in size as files are added
   and/or  removed  from  your system. Selecting this
   option  for  partitions,  such  as  the  /  (root)
   partition, which will have data constantly written
   to it is always a good idea.

This is completely bogus - the option actually causes free space on the disk
to be (evenly?) distributed among all partitions which have this option selected.
So if you select it for a single partition, it will use up all remaining free space.
It has NO EFFECT whatsoever to the post-install behavior of the system.

The description might possibly lead users to false sense that the partitioning
layout doesn't matter much, because size of the partitions will be
automatically adjusted when needed.