In his watershed 1896 essay "The Tall Office Building Artistically Considered," proto-modern architect Louis Sullivan famously declared, "form follows function." He actually wrote: 
It is the pervading law of all things organic and inorganic, of all things physical and metaphysical, of all things human and all things superhuman, of all true manifestations of the head, of the heart, of the soul, that the life is recognizable in its expression, that form ever follows function. This is the law.
To Sullivan, "function" didn't mean merely "utility" or "pragmatic use." Instead, it meant something like "life force." His "form follows function" dictum expressed a kind of essentialist vitalism. The "essence" of a thing in nature (an eagle, a cloud, a river) is its life force. This life force results in the outward form of that thing. In Sullivan's own words, "Unceasingly the essence of things is taking shape in the matter of things." To Sullivan, the essence of a skyscraper is to be grand and tall. But grandness and tallness are hardly the "function" of a skyscraper in a modern utilitarian sense.
Sullivan's "form follows function" dictum was famously adopted as a core mantra by Ludwig Mies van der Rohe and has remained a cornerstone of modern and minimalist design to this day. But "function" to a contemporary usability expert has come to mean something qualitatively different than what Sullivan meant by "function" in 1896. Consider the examples that Sullivan uses from nature to illustrate his law:
Whether it be the sweeping eagle in his flight, or the open apple-blossom, the toiling work-horse, the blithe swan, the branching oak, the winding stream at its base, the drifting clouds, over all the coursing sun, form ever follows function, and this is the law. Where function does not change form does not change. The granite rocks, the ever-brooding hills, remain for ages; the lightning lives, comes into shape, and dies in a twinkling.
A number of problems arise when this passage is read from a utilitarian perspective. What exactly is the utilitarian function of granite rocks that necessitates their particular form? Is the form of every single passing cloud dictated solely by its particular utilitarian function? If so, then all clouds of the same type would have the same form, since they all serve the same behavioral function. Yet each cloud is unique and ever-changing. Is the form of an apple-blossom really dictated solely by its pragmatic function? When "function" is read as "use" rather than "life essence," Sullivan's "law" is disobeyed in the natural world.
When "form follows function" is invoked as a principle in the world of web design, it is often invoked by aesthetically handicapped non-designers (information architects, usability experts, human/computer interface consultants, Ajax programmers) as an excuse to make visually bland websites. But from a utilitarian perspective, the "form follows function" rule doesn't inherently lead to good design.
The real "art" of design lies not only in accurately assessing the functional requirements of a project (the easy part), but also in developing the forms most suitable to those requirements (the tricky part). In the hands of a master designer like Charles Eames, who was already a genius at achieving elegant formal solutions, "form follows function" usually led to good work. In the hands of non-design-savvy web programmers who abuse the principle as an excuse for blandness, "form follows function" usually leads to mediocre work.
Every Force Evolves a Form
One problem with "form follows function" is that it is tautologicalit presupposes that every form in the natural world exists as it does because of functional requirements. We start with the end result (the form), look backward toward its origins, and assume that the results were inevitable. But there are any number of reasons why something might have a particular form (chance, malevolence, whim, purposeful design, play, folly, and numerous combinations thereof).
A better, less tautological mantra comes from Mother Ann Lee (173684), founder of the Shaker movement in America: "Every force evolves a form." From this perspective, form doesn't simply, dutifully follow a set of functional requirements. Instead, dynamic forces gradually forge resultant forms. These forces aren't simply functional; they can also be communal or spiritual, as was the case with the Shakers.
For working designers, "every force evolves a form" is a more useful rule. The design process actually begins with something that doesn't yet exist but needs to exist, and it moves forward toward a formal result. Function alone doesn't drive the resultant form. The form evolves from the holistic forces of the projectaudience needs, client desires, ethical obligations, aesthetic inclinations, material properties, cultural presuppositions, and yes, functional requirements. "Function" is rightly seen as a single, isolated, quantifiable aspect of the overall "force" driving the form.
True, the Shakers did esteem utility. They found it beautiful. According to one of their slogans, "That which in itself has the highest use, possesses the greatest beauty." But more forces were bearing on the form of Shaker furniture than mere utility. The teachers of the Bauhaus also esteemed utility, but the forms of their furniture are far from identical to the forms of Shaker furniture because a host of other historical, philosophical, and material forces in addition to mere utility were affecting and evolving both forms.