For example, suppose that someone took some pre-Rennaisance paintings and improved them by correcting some of the technical mistakes in their use of perspective. Although the technical delivery would be improved, and quite possibly even their aesthetic appeal and power in some cases, they would have considerably less artistic/cultural value. Their significance would immediately become questionable, and it would be impossible to learn anything about art in the time period in which they were first rendered, since an observer would not know which aspects were actually from that era and which were added later.
I realize that with Star Wars, the differences are much more minor. But 20 years is a very long time in the rapidly expanding technology of Hollywood. The fact that it was Star Wars that largely begat this explosion of special effects realism makes it hugely important to preserve it in its original form for cultural/historic reasons. This is why I am so disturbed that film institutes such as AFI haven't been screaming bloody murder about the lack of preservation of such a groundbreaking film. In fact, NO film even of moderate interest should be allowed to be lost or diluted in this manner. It should be a cultural no-no, period. It has nothing whatsoever to do with what I like.