For example, there's a property for buildings that essentially seems to say "this one comes free from age x onwards", and it contains the game eras (ANCIENT, MEDIEVAL, RENAISSANCE, INDUSTRIAL etc.).
This property doesn't seem to do anything for the human player, but I have the suspicion AI players get those buildings or maybe even just their effects for free starting at the cited age.
For example, iirc the Colliseum has this set to RENAISSANCE, so any city the AI acquired once it is in the renaissance would automatically come with a free Colliseum, which would help immensely, especially when they go on a rampage gobbling up a huge amount of cities almost at once.