The Safari Zone is the second road to everything the region does not otherwise offer. Its pool is built by subtraction: any species that is not on a route, not a fossil, not in the Sanctum rotation and not a roamer goes in here.
That makes it the only way to most of the Pokédex — 675 species, none of which overlaps with the lists on the other pages. The pool rotates, so not all of it is available at once.
A Safari visit is bought and timed: you get a set number of searches rather than an open field, so it is worth knowing what you are hunting before going in.