I'm not even close to being an expert on this, but I believe air rights can be sold to the owners of neighboring properties, who can then build taller buildings than they would otherwise have been allowed to build. In the case of the Imperial, it seems plausible that the sale of air rights might be related to the skyscraper being built on that block along 8th Avenue, but that's complete speculation on my part.
Maybe someone with more expertise can give a better answer!