To answer your question directly—although I would love to have chimed in on the previous speaker—we restore buildings for three different reasons. One, because of architectural excellence. In those instances you don't want façadism. Two, we do it for scale. Sometimes those historical buildings are in a row, therefore really the facade may be all you require. Third, we restore them because they may be associated with some famous event or some famous person who lived there.
Circumstances alter cases. What we don't like is preservation that preserves a state of architectural decrepitude. This is morphing into your question. We want to bring the buildings into the 21st century. We want them to be used; we don't want them to be museums. We want them to be representatives from living pasts. Sometimes what is more important than the actual bricks and mortar is the spirit that informed the creation of these buildings in the first place. We get these old industrial buildings that are 150 years old, 120 years old; and the optimism and the love of fine work that informed the kind of work to build an industrial building 150 years ago was quite incredible. There you really want to be careful what you preserve.
In direct answer to your question, you may be, for example, required by the building code to provide earthquake protection. In a new building, you build that right into the structure. When you're restoring an old building structure there's no way to beef up each column, so now you may have to strap new columns onto existing columns. You may have to put some kind of bracing all along the exterior wall. You have to somehow avoid the windows in doing that. It can be very costly.
I mentioned, for example, that the building code may have certain insulation requirements—