What do Non-Executive Board Members bring to the Party?

News that hospitality veteran James Horler resigned from Patisserie Valerie's board raises the question: what purpose to outside board members play? If they are excused from supervising the nitty-gritty of a company's accounts are they then left with being a sounding board and in-house giver of strategic advice? And who are they accountable to?
(16-Jan-2019)
James Horler resigns fro Patisserie Valerie Board