I'm not in the bioinformatic domain but maybe there are some legacy tools that depend on some specifics of gzip.

I was thinking something like the possibility to seek through a compressed file (after having built an index). Your DNA file is probably stored on a shared folder somewhere (if it's big you probably don't want to copy it to every workstation) and you point your software to the gz file directly, it create a seek-index on the first use, and then when you need view only a small section of the file you don't have to extract the whole file.

Some more advanced compression like zstd might use some form of dictionary to do the compression/decompression, which may be bigger (up to 32MB? max dictionary size for zstd whereas lz77 used by gzip use a dictionary based on a sliding window of the last 32K token) which you may have to transfer.