First, the return!
Put it simply, because you want to chase an extraordinary return. With public markets, your return (if there is any) is typically limited. However, you could strike gold if you are an early stage investor for the next Google or Facebook!
Second, diversification
If you have a large portfolio, you need diversification, and private companies' performance tend to have little correlation with public stock market's performances.
Third, expertise
You will be in the driver's seat by diversifying to industries and companies of your choice, preferably in areas you have expertise.
Next, we will discuss what are the downsides of investing like a venture capitalist.