Over on the lovely Pink Air, Jeffre Jackson remarks upon the popularity of gel buttons on the intermenet. He believes they have become ubiquitous because they contain all kinds of reality triggers. To steal his beautiful phrase:
They're artificially supersaturated with visual reality, so they're especially fascinating in the arid, 2-D environment of the web savannah.
All of which makes a lot of sense. But I think there is an additional factor that explains their popularity: affordance.
To steal from the Wikipedia:
An affordance is a property of an object, or a feature of the immediate environment, that indicates how to interface with that object or feature. The empty space within an open doorway, for instance, affords movement across that threshold. A couch affords the possibility of sitting down on it.
So inherent in the image of a button lies the means by which you interact with it, which is a remarkably useful quality when attempting to build navigation into a web page.
How does the button communicate this function? It must be using some kind of language.
By understanding how objects communicate their affordance we gain a better understanding of how people parse the world around them. If we can understand this language we can build tools , and brands, that explain their own function.