"No sadhana succeeds without Ganpati sadhana completed first" is repeated so often in these discourses that it stops sounding like a rule and starts sounding like a law of the system. The reasoning given is specific, not ceremonial: early sadhana activates the muladhara chakra, and that activation brings a surge of kaam and krodh before anything else. Ganpati is named as the devata who holds authority over exactly that chakra — bring him to prasannata first, and he keeps that surge under control through every sadhana that follows. Skip him, and the discourses hold that obstruction will surface eventually, sometimes only at the very last stage of something you spent months building.
That's also why Ganpati sadhana sits at the very base of the three-tier structure covered elsewhere in these discourses — not a devotional add-on, but the actual foundation everything else stands on.