The human mind can't comprehend a stream of data any more than it can "see" an electron. It needs a way to interpret the incoming data as something meaningful. So Netrunners use an interface program -a super-advanced version of the more primitive "virtual reality" systems of the 1990's -to interpret for them. The interface intercepts data coming through the cyberdeck and translates it into something understandable - then routes the altered data to the Netrunner's eyes and ears. The world perceived through the interface is real, because it directly plugs into his senses.
So why go through all the trouble to create interfaces? Why not just use a keyboard like the rest of the meat minds? Partially for the fun of it. But in addition, a realistic and dangerous interface gives the Netrunner an extra edge. It keeps him alert, involved and interested in his environment. After all; what would you react faster to - the word Demon appearing in the air in front of you, or a living, breathing, five-ton monster cracking a flaming whip over your head?
The early interfaces were an art form; millions of programming hours were devoted each year in constructing accurate and interesting realities for Netrunning, using sophisticated artificial intelligence programs and random story generators. These interface programs functioned on a low end, narrow focus bandwidth, which could not carry much more information than an old fashioned computer modem of the 1990's. In addition to being limited in scope, these early interface programs were also unable to give the Netrunner a sense of his position in the real world beyond the computer screen.
Then, in 2014, the wizards of the Net achieved a major breakthrough - the Ihara-Grubb Transformation Algorithms. The I-G Transformations allowed a cyberdeck to extrapolate the pathways of the Net in relation to their "Realspace" coordinates, then generate a graphic model that could be perceived by an interface program. The results could be used as a navigational aid through the Net, as well as providing a sense of space and time not possible with earlier designs.