For me this is the best reason(I'm just talking about those legit one that could succeed). ICO is the place for sharing new ideas: You only have to look at the progress of Ethereum to decide to invest in an ICO. All the cryptocurrencies that have become popular today have had the backing of an ICO.
Community building: By investing in an ICO you will become a part of a community and you will also come to know about the forthcoming projects as soon as the intention is revealed. You will thus have the first opportunity to invest in.
Access to the valuable tokens: By investing in the proposals at the early stage you will receive tokens that you can further invest or trade. Thus, you will earn immense profits after it is out in the market. Though currently only few ICO could survived or could get their softcap because there are a lot of ICOs out there and a lot of scammer this day.