What does a good non-executive director actually do for an agency?

I'm considering a NED but I'm not sure what I'd actually get

A good NED brings outside experience and challenge, holds the executive (including you) accountable, helps with the big strategic and governance decisions, and acts as a sounding board and mirror for the founder. At this stage they also help professionalise the board, reduce founder isolation, and prepare the business for whatever comes next, including an exit.