One of the most famous dictums associated with Modernist design is that "Form follows function." Typically, what modernists mean by this expression is that the design of a product should be dictated by the job it is to perform for users—and nothing else!
However, Modernists tended to take a narrow view of what a function is. In their view, this was limited to physical services that a product might perform for its users.
Deliberately omitted were interpersonal services, such as impressing your friends with your good taste, or conveying your sense of identity to casual onlookers.
This issue is especially prominent in the case of wearable items such as eyeglasses. As this brief but nicely illustated post by Katherine Schwab notes, eyeglasses developed from a purely functional item to a fashion statement.
In terms of immediate physical services, eyeglasses correct vision problems. However, if that were all there is to eyeglass design, eyeglasses would likely all look about the same. Anyone who has visited an optometrist will realize that they do not.
Because eyeglasses are highly visible to others, they have significant social utility as well. A person's choice of frames and colors helps to convey what social group they belong to or what personal attitudes distinguish them, for example. In other words, eyeglasses can be cool, nerdy, hip, retro, etc.
Think of the recent hibernation of Google Glass. This head-mounted computer was withdrawn from public circulation not because it did not physically work but because wearing them was too socially awkward for most users.
Of course, as social norms change, attitudes towards computerized glasses may change too. In that case, what will they look like?