Well, obviously to harass and intimidate (e.g., release employee names and their email addresses), but … well, here's the "official" reason from the hackers:
"We've noticed quite a lot of attention has been diverted to a supposedly malicious organization known as Planned Parenthood. The actions of this 'federation' are not seen as right in the eyes of the public. So here we are, the social justice warriors, seeking to reclaim some sort of lulz for the years and thousands of dollars that Planned Parenthood have wasted and made harvesting your babies."
Which is about as coherent as your typical 419 scam email, to be honest. Which makes one wonder who these mystery hackers actually are (or are pretending to be).
Anti-abortion hackers attack Planned Parenthood, release databases, employee data
Planned Parenthood is having a rough month.
Based on that statement; 4chan or 8chan.