Earth is described as “gone” by Darwi Odrade in Chapterhouse, but the Dune Encyclopedia describes it as once depopulated (possibly asteroid stike) and then re-populated and the ultimate source of the Zensunni.
The plot significance is that worms can be transplanted to other planets, I guess.
They’re guiding them, but they have FTL engines (Holtzman drive). They’re not just plotting a course ahead of time, they’re actively steering the ship (Steersman is an alternate title for a Navigator).