So there's the Sprawl, which is what we would probably term as real, and there is the matrix, which we would term as virtual.
"Cyberspace. A consensual hallucination experienced daily by billions of legitimate operators, in every nation, by children being taught mathematical concepts... A graphic representation of data abstracted from the banks of every computer in the human system." (N51)
Two separate entities, one of them the meat world, the other, the world of the mind. And yet, can they really exist independently of each other? Could the Sprawl exist as it stands without the matrix to back it up, to provide the backbone, to provide for the inhabitants of the Sprawl? And could the matrix exist without the Sprawl, without the pool of people looking for a way out, looking for a way out of the world where a good night's sleep is lying in a 3 meter box?
Note too, the Sprawl, true to its name, has very few distinguishing characteristics. There are a lot of buildings, but that's pretty much all we can get out of it. There are a lot of hotels and bars as well, and you can get from one part of this expanse to another rather quickly. But all things considered, one part of it might be exactly like the others. The matrix, however, does have location. Case interacts with the "ice belonging to the New York Public Library" (N56), and we see that all of the structures in the matrix, as virtual as they might be, actually do have some sort of defining characteristics, and they are tied to some actual place. We don't just have random information floating around in cyberspace, there is actually some sort of structure.
Of course the ice that Case is looking into is tied, in this case, to a structure in the physical world. Where the defining characteristics of the real world have faded into just an endless mass that is called the Sprawl, the matrix has risen up to take the place of the structure that was lost.
The people need the matrix to escape the Sprawl. They need it to find information otherwise inaccessible to them in the Sprawl. The cowboys need it for their livelihood. And the people need the meat world for everything still associated with being human. They need it to eat, for even Case, after being jacked in for days, still needs to get up to deal with his physical body.
Alone, the Sprawl is a pretty awful place to be. Alone, the matrix has no reason to exist. Together, they create some kind of bearable world.