Basically it allows you to start your item count right away, but you do not enter a leaderboard until the game connects to its servers (aka airport mode on)
So for example, the fairly recent doll mini event where we collected 10 items every 4 hours. I collected 4 times (40 items) in airport mode on the 5th time I played with airport mode off so I entered the leaderboard with 50 items after I collected the 10 and I had a 20 item lead on the leaderboard. Now you may wonder why that gives you an advantage? The game places X number of players in a leaderboard and when that fills, the game starts another leaderboard. So the later the leaderboard, the higher you should rank, if the game counted all the items you collected in airport mode.
The disadvantages with playing in airport mode is your unable to collect daily gems, rewards and I believe you loose your consecutive day count? Not sure on that though, because I never go a full day in airport mode.
Whether or not you consider this cheating, or want to play in this mode is up to you. I don’t do it very often, but if I know my playing time is going to be limited during a mini event, I’m more likely to use airport mode.
I finished 6th, but I DID NOT play in airport mode. Think I finished in the 10 50 for the doll mini event that I mentioned above though.