It's a code requirement in California for any dwelling space.  Every 
house
I've lived in has 2X4s on 16 inch centers.  My great-grandfather built 
all
houses this way.  My house now was built in 1925 -- 2X4s on 16 inch 
centers.
The 1906 house I lived in had 2X4s on 16 inch centers.  The Victorians 
I've
worked on had 2X4s on 16 inch centers, so the standard has been around 
for
a very long time.  Fiberglass insulation specifically for walls comes in
16 inch wide rolls.  Roof insulation comes in 24 inch wide rolls.

Why?  You'd need to ask a structural engineer for the rational.

Mike in Sacto