[Arryn]

1. Pushing your dominion into lands controlled by your enemies reduces the benefits that they get from their scales. Often, this hurts them much more than your dominion helps you directly. And anything that hurts your enemies is usually good for you.

2. The stronger your dominion the greater the effectiveness of very powerful and useful spells such as Gift of Health. There are also spells that can only function in your dominion (such as GoH).

3. If you push your dominion so much that none of an opponent's dominion remains, their pretender goes 'poof'. This can be a much easier victory in some cases than trying to defeat an enemy that is strongly-entrenched in their capital castle if that is that enemy's Last remaining province.

4. Dominion spreads TWO provinces out from temples, not just one, so when the enemy builds a temple in their province directly across the "border" from you (which the AI does a lot), they are pushing their dominion deeper into your turf than you may think.

5. You need temples to recruit priests (and sacred troops).

Building temples not only helps you to accomplish what I've said above, but it counters the enemy doing the same to you. And they will. Count on it.