It's two things really.  You want it to absorb water that ends up in
the
system, so that there aren't small pockets of water that cause rapid
localized corrosion.  Silicone doesn't mix with water and water that
ends up in the system ends up in the low points.  Silicone doesn't
attract water though.

However - ordinary glycol-based brake fluids are hygroscopic, meaning
that they will absorb (actually attract) water vapor from air.
Different
brake fluid formulations are better or worse in that regard.  I
remember
hearing about a demonstration where someone took a sealed bottle of
brake fluid in a humid climate at a racetrack near Atlanta.  He then
poured the fluid into a shot glass up to the top.  After an hour, the
fluid had spilled over simply because it attracted and absorbed
moisture
from the air.

Apparently ordinary brake fluids have a certain shelf life because a
small amount of moisture can permeate the plastics (or even the
plastic seals of metal cans).  Once opened, brake fluid should be
used quickly - maybe less than two weeks to top off after a brake
job.  No brak system is perfectly sealed, so moisture will be attracted
into the system by the fluid.