As NGF says : dominion spread is quicker when dominion level is higher, there's no such "rule" than "max territory first then spread", both are done simultaneously .
So the only remaining use that I know of a low dominion are
- to voluntary *slow* the spread of crappy scales you've taken (for example mandatory Sloth of some themes)
- to get unnoticed when you sneakily conquer "distant" provinces, Gandalf said it used that as a strategy
- to have more design points for other things - Pretender, scales, castle..