Why? To become rich duh.

I think you need to ask yourself: is the project useful and does the fact it's using blockchain technology give an advantage to the project. Trustworthy team? Big supply? Support? Dev? Etc.
And be completely honest with yourself : if it's to good to be true, it's not true.
If all these conditions look great, I invest a bit and consider my coins to be just like stocks in a real life company.
I use exactly the same principle for ICOs.
It has been working out great for me.