From what I've been reading about Pre-Sales, they are primarily designed to get a financial influx ahead of an ICO - to cover costs like hiring additional staff, covering the costs of getting the ICO ready. Some ICO's with Pre-Sales have been run really well, while others have had their challenges. One of the big problems that some projects faced when running a Pre-Sale prior to the ICO, is the Pre-sale price of tokens is often significantly cheaper - and a lot of projects don't enforce any restrictions on when those initial tokens can be sold.

So what has happened with some of these situations, is the early investors who got in at a discount, then sold their tokens when the tokens initially list - generally at a price higher than they paid. This is seen as a safe and guaranteed return on their money without really risking market volatility. This is a reward for early investors, but can have a negative effect on the price of the token once it hits exchanges in the early stages. It's never a good look for the price of something to tank once it's available to the public.

It's an interesting topic and there's so far been a lot of different variants attempted, with varying levels of success. ICO's like Canya and INS have been run intelligently to prevent this kind of dump of tokens early, in that they offer significant rewards for those who buy early, and hold, by using periodic airdrops to those wallets found to still contain coins from the ICO or Pre-Sale phase. Free tokens for hodling is a fairly lucrative incentive to hold after all.