Disadvantages when a company’s general counsel serves on some other company’s board have been covered here (See my post of May 11, 2011: conflicts, time, fees.). Those drawbacks notwithstanding, it is not uncommon to find general counsel who serve as a board member elsewhere.
The advantages that come to mind include these. (1) A general counsel rarely has a career path up or to the side in a company, so the opportunity to step into a somewhat different role with another company provides a form of mobility. (2) The most common driver is probably morale. If a general counsel has a passion for whales and wants to serve on the Save the Whale’s board, wonderful! (3) A broader understanding of business and making money comes about through exposure to board-level debates and decisions. For a general counsel, the view is farther and deeper from the board room. (4) Sometimes, like arranged marriages, it serves both businesses to have board-level linkages.