the 'Function' of a product involves more of an element of the
performance than does considering the purpose behind its creation.
An item can be said to be 'functional' if it capable of doing
the task for which it was designed - that is fulfilling the purpose
behind its design.
The 1940's saw a series of 'functionalist
designs' - This was a time when consumers had little income
that could be given over to acquiring products that did more than
satisfy the need for which they were designed. After
all it is a bonus if a table or a chair looks good and feels good.
If it merely performs the function of supporting objects at a
suitable height (say for writing, reading or eating meals) then
a table is functional and its cost might be expected to be less
than something made of exotic and expensive materials.