Question about seekable format and checksums

The seek table of the seekable format has an optional `Checksum` [field](https://github.com/facebook/zstd/blob/dev/contrib/seekable_format/zstd_seekable_compression_format.md#seek_table_entries) for every frame, where the least 32 bit of the xxh64 hash of the uncompressed frame data are stored. However, the `Content_Checksum` field of a [zstd frame](https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#zstandard-frames) seems to have the same purpose. I there a reason for the seekable format not to use the `Content_Checksum` field with `ZSTD_c_checksumFlag`?

Also, how should I understand the contrib directory, especially the seekable format? Should I take it more as a proof of concept of seekable compression with zstd or more like a feature? I'm just asking out of curiosity.