When you're a member of a board of a company which is traded on the stock exchange, there are all kinds of rules like not being able to share the insight you have on what's happening in the company, you're not allowed to buy additional shares of the company, etc.
Through declining being on Twitter's Board of Directors, Musk can be transparent in talking about what Twitter is doing, the good and the bad. And more importantly, and very scary for the left-wing media, he can pull buyers together and buy as many shares as he wants and take over the company if he sees fit. This will keep all the liberal shareholders of Twitter and the management team, on their toes wondering what Musk will do next. He's the man!