I suspect postgress just brings efficient queries. My initial thought was how fossil uses sqlite as a backing store. but... not only is sqlite intentionally designed as an interchange format(stable specification). the postgress disk structure is intentionally designed to not be a interchange format(they reserve the right to change it at any time) so not that.

So the only real reason is you already have a postgres server and want the efficient query indexes.

As an interesting side note. I found this document on the internal data structure of fossil. https://fossil-scm.org/home/doc/trunk/www/fossil-is-not-rela...