The point of the Air Balloon, as its effect would make you think, is to allow the user to become immune to Ground type moves. However, since the balloon pops, its effect is limited to switching in on Ground moves, and forcing the opponent to use a move that isn't Ground in order to pop the balloon. I'll explain the benefits of these effects now:
Switching in
This is quite simple. It gives a Pokemon that is ordinarily afraid of Ground moves, let's say Excadrill, a chance to switch in on a Ground move with no fear to being knocked out.
This can result in set-up opportunities; for example, if there was a wall only using a Ground move to avoid being Taunt bait, then that wall would have no way to attack Excadrill and pop its balloon, which could potentially result in a set-up opportunity (unless it is something like Hippowdon that carries Roar).
It is also nice if the Pokemon is using a Choice item like a Choice Scarf; it would be locked into the Ground move and would be forced to switch as a result, which you can then predict and act accordingly.
Forcing a certain attack
If a Pokemon can't use Earthquake, then it would have to use a different move to pop it, right? This is exactly what you want. Take that same Excadrill for example; if it was up against a Pokemon, let's say a Double Dance Landorus-Therian (that uses Rock Polush, Swords Dance, Stone Edge and Earthquake), then it would be forced to use Stone Edge to pop the balloon, which could give you a chance to use Swords Dance and then annihilate the Landorus with a powerful attack.
The opponent may also switch, so consider that when you are choosing what to do.
Porygon2 is not a very good option for an Air Balloon, as it is not really afraid or Ground moves making it a bit of a waste when you could use an Eviolite, a much more logical and useful item for it to use. Some notable Air Balloon users are Excadrill, Heatran and maybe Tyranitar is it isn't using Assault Vest or Tyranitarite.
As for the missing thing, the answer is no. The Air Balloon's only effect is making the holder immune to Ground moves until the balloon pops.
