In many cases, it's an artificial distinction. Sometimes form spectacularly doesn't follow the obvious function--take for example the Oscar Mayer wienermobile--clearly, the shape of a hot-dog is not the most efficient to fulfill the ostensible automotive function. (That thing's got to be a bear to get into a tight parking place. . . :-) )
In reality, though, the form is following a 'meta-function'--in this case, the function of advertising for Oscar Meyer. In that regard, it's highly effective, as the fact that it popped into my head as an example testifies!
(Even more so, if you find yourself getting hungry right about now. . .)