What is the point of building a fortification or fortress?
What is their effect in the game?
I really wasn't able to tell by looking through the manual and the editor--other than the fact that they can be destroyed because they have structural points.
Do they just increase entrenchment, or do anything else?
Yeah forts are good, especially the 5 PP one. They force your opponent to either use lots of artillery or heavy bombers to destroy them. They are only a bit expensive, so finding a good line with lots of natural obstacles so that you only need a couple of forts is often not easy.