For archival purposes, you want a lossless master so that new copies can be made from the original using any new codec that is later developed, whether that codec prioritizes quality, file size/bitrate, encoding/decoding cost, certain hardware optimizations, etc.

That way a 1995 film can be shown using a 2035 codec when the time is right, rather than relying on a 2035 encoding of a 2020 encoding of a 2000 digitization of a 1995 analog master.