The Necessity of the Golden Path
Herbert doesn't explain why the "Golden Path" (Sachr Nabai, if memory serves) until "God Emperor of Dune" in the scene where he takes Siona Atreides (a very distant descendant of his long-dead sister-wife Ghanima and her consort Prince Farad'n Corrino) in to the Sareer (his pet desert) for her "testing."
Both Paul and Leto II's prescience presented them with a horrifying fate for mankind if the future weren't radically and permanently altered. Presented in a vision that Siona has during her spice trance, she sees a race of machines hunting down and systematically exterminating humanity--in "Terminator"'s Skynet's style.
The personal sacrifice that was the only thing that could insure humanity's survival was one Paul refused to make. Leto II, on the other, made of sterner stuff than his father, embraced the choice; thereby paying an almost unendurable personal price. It's easy to spend 3,500 hundred years trapped inside an alien body to save your own family; but to save generations of humans you'll never see? It's easy to imagine why even a revolutionary and an Emperor would quail?
The only way to avoid this fate for humanity is two-fold:
1.) Humanity must be scattered throughout the universe where nothing and no one can ever find all humans again. Humanity would thus be invulnerable to genocide. If you can't find every member of a species, it can't be wiped out. This is the sociological component of the Path: turn the Empire into a vast zoo and turn the screws ever tighter until, when the lid is removed, the explosion is uncontainable. I won't say anymore to avoid spoiling the books after "Children of Dune."
2.) The one way to ensure against prescient detection* is to create a line of humans who would be invisible to prescient detection. Leto II achieves this in Siona--his wild card and the reason that he took the breeding program away from the Bene Gesserit. Even he can't "see" her, though he is able to perceive her absence in some situations (in one scene he can see her footprints in the sand). The point is, Leto has introduced a truly new element into the human genome, setting human evolution back on the natural course that melange ("spice") had disrupted.
Aside from reading the REAL Dune novels--and NOT the trashy garbage his son has written to prostitute his father's achievement for sheer greed--"The Dune Encyclopedia" (ed. by Willis E McNeely). Sadly, it's out of print, though it can be found used (sometimes at very steep prices; none-the-less it's an absolute must for Dune-o-philes).
*such as the genocidal machines of the Golden Path's vision, and presaged, albeit in proto-typical form, in the Ixian Navigation Machines in the last two REAL Dune novels or even some potentially sentient alien race.
"The greatest trick the devil ever pulled was making the world think he didn't exist."