It is all about natural risk. You can buy tokens on exchange but there will hardly be any gains, or little. Sure but not much profitable. At ICO you have some risks but you can gain something, plus bonuses
You should never consider bonuses when investing in a coin, many people get blinded by it and invest in a project by taking a rush decision when they could have used that time to study the project instead and if they did that they would have never invested in the first place, so forget about the bonuses in fact the bigger the bonuses the biggest the chance an ico is a scam.