The use of a hardener may help to prevent fungal attack of your negatives. Read this paper from Kodak.(posted 8482 days ago)
My own experience tends to bear this out.
Ilford, on the other hand, have completely changed their stance on hardener in recent years, and now completely eschew its use, apparently. My old 'Ilford manual of photography' strongly recommends the use of hardener, but Ilford have given no explanation for their recent volte face.My collection of negatives of 30+ years have come to no harm from the use of a hardening fixer, so I, for one, will continue to use it. It certainly doesn't seem to do any harm.