"The form of a building must follow its function" means that the beauty of the building, the cost of the building, the local trends in architecture should not matter when building.  The architect and the client should decide what the building is going to be used for and where the building will be located.  The building should then be designed for the use of the building.  An example of this concept is when the Greeks used scroll work and carvings to hide the rough joints in the wood and stone, when the building went up.  With the inventions of today's building materials we no longer need to hide raw joints.  We can bend glass and steel.  Why do we still put fancy scroll work, and fancy statues on buildings?  They have nothing to do with the function of the building so they are unnecessary.  Too many people build their homes to impress others rather than to suite their lifestyles.  Why do we need fancy Victorian architecture in a society that builds with brick, steel, and glass?