Directors & Officers liability protects the people who run the company. When a shareholder, investor, employee, competitor, or regulator alleges that a decision made by management caused harm, D&O pays defense costs and settlements. Without it, those costs land on the individuals personally, and on a balance sheet that was supposed to fund product.
It is structured in three parts. Side A protects individuals when the company cannot indemnify them — typically because it is insolvent or legally barred from doing so. Side B reimburses the company when it does indemnify. Side C, entity coverage, protects the company itself for securities claims. For a private venture-backed company, Side C is usually limited to securities-type matters, which is exactly where funding-round disputes live.
Practically, D&O becomes non-negotiable the moment you have a board seat held by someone who isn't a founder. Experienced independent directors and most institutional investors will ask what your D&O tower looks like before they sign. It is often cheaper to buy than founders expect, and dramatically more expensive to need and not have.