While the Enterprise may be a space shuttle only in name — it's not actually capable of spaceflight — it was the prototype craft that made the rest of NASA's shuttle fleet possible. Even without the engines to get to orbit or the heat shields to survive the journey, Enterprise completed a series of crucial Approach and Landing Tests.
The whole point of the shuttle program was that it would provide what no other space agency had before it: a reusable spacecraft to ferry humans to and from orbit. If we could launch a shuttle up like a rocket, Enterprise proved we could land one like a plane.
NASA wants to have all the shuttles carted off to their new homes by the end of the year. Last week saw Space Shuttle Discovery carried to the Smithsonian, and over the course of the next few weeks the Enterprise will be installed at the Intrepid Sea, Air and Space Museum in New York City. There's still Endeavour and Atlantis left; the former will be flown to Los Angeles this fall, while the latter will stay in Florida and be installed at the Kennedy Space Center sometime before it kicks off an exhibit unveiling there in July of next year.
Curious parties can keep up with the retirement of the shuttle fleet — including where to see the craft in which museums, or how to spy them during transport — at this NASA website here.