Marketing strategies are unique in themselves and there's no single best strategy out there, but a combination of them all gives the best result. There are times when some projects did not do any public sale, but just private sales. They instead give airdrops in form of bounties and also give airdrops to their private investors. A classic example of this strategy is UCHAIN.

Airdrops help to create more awareness about a project and make them more popular. The much touted notion that those who receive tokens from airdrops tend to drop them at the exchange and thereby cause in the price might be true, but also remember airdrops are usually a tiny percentage of the total supply. There's a set of token holders who tend to be ignored simply because they contributed financially to the project, and these are the private investors with huge bonuses without any lock-in period. These guys have been notorious for dumping their huge bonuses at the exchanges too, thus crashing the price of the tokens massively.