Soy decreases allergenicity when fermented, which might be meaningful if you are allergic or intolerant to soy.
But less well known, some are allergic to fermented soy but okay with soy.
There is a study on late onset anaphylaxis with natto allergic. Unusual in that it is IgE mediated but triggered only when it reaches the intestines, taking a mean of 8 hours.
I am not okay with soy but my reaction is much worse with raw fermented soy. I suspect it is the difference between soy protein intolerance versus mold (aspergillus) allergy. If I didn’t have the latter I might have tolerated small amounts of fermented soy, such as that of a typical condiment serving size, which would have opened up some more flavor adventures, alas.