Pre-sale offers cheaper tokens to investors and mostly for big pockets investors. I personally do not like the pre-sale because it's on of the reasons you see dumps when a project hits the exchange. They dump to drive the price down and then buy more tokens
I do not mind pre-sale only if the coins are frozen for sale for a period of about a year. This is the only way to ensure that large investors who buy coins with large bonuses will not sell the coins immediately after the ICO and will not bring the price down.