Can somebody explain the rational behind choosing airdrop distribution method? Why the creator(s) of a revolutionary product chooses this method of distributions? 
It puts the coins into the hands of many people. This helps create awareness of the coin, and increases the chance that some will invest more heavily into it.
Some people refer to Metcalfe's law (network effect) in relation to this, so you might look it up for more info.
The first airdrop I received was about $10 in flash tokens for using some app on a blue stacks android emulator. I started to accumulate eth-20 tokens, and I really won't touch anything that requires a download anymore. I didn't see them priced anywhere for well over a month, and I accumulated about 5 different tokens. About a week ago, someone showed me that one of them was on an index. Great, 40 cents total out of 2500 tokens. When I added it to an online portfolio, I learned that it wasn't 40 cents, but something like .40 ETH-and substantially more a week earlier. Someone recommended etherdelta here last night, and I saw the price of other free tokens i accumulated. Another one I got for free has been trading 1m for 1.7ETH; they gave out 5m to each user.
i can understand why the coins were given out for free, but I can't for the life of me understand why why some of them are trading for so much. As nice as it is, it makes me feel a bit uneasy. Can anyone explain why these eth-20 tokens have any value, and why anyone would pay for them? Or any reason to hold onto them? I mean, it's not like you are going to go into wal-mart anytime soon and pay with eth-20 tokens.....Unless someday wal-mart makes their own eth-20 tokens.