Silicon Valley is obsessed with them because it's the only place where hundreds or thousands of employees are given significant equity; many of them have $100k - $1 million in vested equity 1-3 years into the job. You don't want that frozen in a private company indefinitely. You want liquidity to use it and to diversify your holdings. This doesn't mean you think the company will fail and you just want to "take the money and run".