The functionalist idea of fitness for purpose was an important part of Frank Pick’s view of design. (See my previous post.) Michael Saler says the phrase comes from Ruskin, but I haven’t been able to find it. If anyone has a reference, I’d be pleased if you could share it with me.
From Google ngram it seems that “fitness for purpose” had scant mentions until the 1920s, then a few until 2006, when usage shot up exponentially following its popularisation by British home secretary John Reid. Its main use isn’t in design at all and it appears a lot in consumer and contract law. And its first use may be in Pugin rather than Ruskin.
On page 1 of Contrasts he says, “It will be readily admitted that the great test of Architectural beauty is the fitness of the design to the purpose for which it is intended, and that the style of a building should so correspond with its use that the spectator may at once perceive the purpose for which it was erected.”