Often you can't reliably re-render shots from a few months earlier on the same project due to the pipeline changing so rapidly. Nevermind going back to the tapes for files that probably came off Irix workstations originally. The file formats for assets (models and textures) are backwards compatible enough that you can usually reconstruct models and maybe animation. So having an archive of the original files doesn't mean that you can re-render at higher resolutions without a lot of effort. The originals would need to be redone with more detail in any case to benefit from the higher resolution.