Well, Ereog's Tower sucks so it only makes sense it got buffed with its own national park.

1. It grants +1 fame/turn. (Fame lets you recruit a hero at set tiers - 2, 10, 100, 200, 400, 800, etc., and is used to build henchmen/sion units)
2. It grants +1 air power.  This is the same for most purposes as having an additional Air shard building, but without the +mana.  The main effect is that it will improve the effects of certain spells, notably Haste, Evade, Storm, Thunderstrike, and Tornado (if so, it will be mentioned in their descriptions, both in-game and in the hieragemnon.)  If you happen to be playing Pariden or a custom Amarian faction, it will also improve your units' dodge by +1.
3. It grants a 1 time bonus to fame when built (+10).

Basically, you probably only want to spend the time building it if you have a need for fame (e.g. if you intend to build lots of Sion or Henchmen units.) or really want to maximize the effects of your air spells.