A wizard who begins a turn in a province that contains Standing Stones has 2 extra magic points to spend on that turn.

Especially if you have a wizard whose combat spells are weak -- so that you don't feel compelled to have him accompany your armies -- you may wish to build Standing Stones somewhere and park your wizard there.

Ulvir will almost always build Standing Stones, since magic is its strength and 2 extra points each turn is particularly useful to it.