What is the advantage of a plains province?

[aww1st]

They have good farming, but since they have negative wood, iron and stone, what advantage would there be to annex one?

[tiger]

They have +20% on farm limits, so they can have more people then other provinces. They could have additional bonuses if they are in 2. or 3. circle. You could annex it to wall some other province, depending where she is on map. And in later stages good player annex all provinces so basicly you will annex it.