[Cohen]

Simply there's no limit where your domain could spread.

Every Dominion Point you get, plus the bonus of the temples (+1 every 5 temples, to a max of 10) gives you the chance that a temple, prophet and god (god counts as 1 dominion point fixed and 2 temple check) add 1 point of dominion.
Having dominion 5 means your temples have a 50% chance of pumping your domain by 1.

The temple try to raise first the dominion where it's located, but it's not sure. The closer the dominion is to his maximum, the more are the chances the temple spread the dominion point in another province starting with the neighbouring.

When all provinces are maxed of dominion, every temple pumping dominion will push your dominion in the enemy border.

Every slave sacrificed count as a temple check.

So on the more temple you've the better it is.

And dominion is usually needed for many strategies, most of them already said, but I resume them.
First you need it to support your pretender and prophet if they're going to operate in enemy territory, so you need dominion push.
Second, immortals. There're a lot of them, Wraithlords, Vampires, and such, they benefit of immortality only under your domain.
Third, your troops get a morale bonus.
Fourth, you get your scales, instead of the enemy ones, and enemy dominion bring unrest.
Fifth, some nation dominion is fearful, like SG or AE Ermor, or CW Pan (they kill your pop), or C'tis Miasma (it disease your non swamp survival troops).
Sixth, many spells work only under your domain.
Seventh, if you've heat or cold scale it benefits your troops, and can weaken enemy troops (like Jotun Niefel Jarls have -2 att and def in heat scale).
Last you can kill people by out dominion them!!!

Probably I'm forgetting something, but it seems a good list to make your domain very important.