Once you’re a certified divemaster or instructor you will need liability insurance just like a doctor has to have malpractice insurance. As a dive pro, you can be held liable for any incidents that occur while divers are under your supervision and, as a result, get sued for negligence or whatever. When that happens, lawyers from the insurance company will help defend you. I have Vincencia and Buckley. You can do a search for them or use the link on PADI’s website. The V&B website will explain their services at length.