Iconography or the design and creation of icons, is an art-form in its own right. The process of illustrating a metaphor as a simple symbol in a way that helps create a more visual approach to a design as opposed to just using copy is becoming for the norm.
The evolution of the art-form has been quite radical as web/UI/graphic designers use them more and more in the design process. They have evolved through multiple iterations and continue to do so even sometimes for no more reason (in my humble opinion) than for the mere sake of it - which is not a bad thing as art is a continuously evolving thing..
Icon design goes hand in hand with user interface design as both are used to create applications and websites that are easy to interact with, making the use of the environment easy to navigate and to avoid creating a negative approach to the product or company through 'over' design or design for the sake of design.
I have been very lucky in that I have been invited to work on quite a few prestigious projects where UI (user Interface Design) and iconography formed the bulk of the work required. One of the projects for one of the big banks in South Africa required creating a large set of icons to be used on bank ATM screens. The idea being that the design of the metaphor in the icon should almost supplant the need for people to read text, especially given the levels of literacy and amount of languages that would need to be catered for.
There are some very talented people out there who are continuously innovating within the UI/icon space and so this art-form will continue to influence digital design platforms (and even print as it all spins off) for a long time to come.