Diversity of perspective, industry experience and or customer access. Think about conflict of interest and broadening your base.
Industry experts become progressively less useful over time
In some instances if you hire a board member from a key customer the deal flow can dry up due to conflict of interest. Consider the person for an advisory role.
A mentor/coach that you can work with in a safe zone.
One or two founders on the board usually it is the CFO and the CEO then eventually but not always becomes one founder (CEO).