We've known for a long time that the Earthen were essentially the 'architects' of the titans work forces, they built things and excavated and guarded their deeper facilities.
Then mechagnomes handled the "Tech" in the titans magitech, while Vrykul were their 'warriors' handling the frontline fighting and Keepers were the Commanders with a wide variety of skills.
So what about the Tol'vir and Mogu?
Why did they guard the facilities in uldum and Pandaria rather than members of the other 'seed' races? And what about the Tol'vir that were in Northrend that were conquered by the Nerubians?
Cursory lore doesn't make the Mogu's original skills or responsibilities seem any different from the Earthen or Mogu, save that they might be a sort of... combination of them.
Tol'vir's responsibilities similarly don't seem far off from the Mechagnomes.
But looking at what we've seen in the N'zoth patch for BFA and the Shadowlands info regarding anima I'd like to posit a theory.
Now, reasonably we know the "out of universe" explanation for all this is... "Because Blizzard hadn't made Mogu / Tol'vir races yet, but wanted to do something that looked different for new titan constructed races" but...
What if the Tol'vir's job was to handle the 'magic' end of the Titans magitech? Given how when they're corrupted by old gods as Obsidian Destroyers or things like "Maut" and "Moam" they seem to have anti-magic skills.
And what if the Mogu's job was to handle 'anima' and spiritual concerns for the things titans created? Or trapping the spirits of Old God minions in some way?