@acdw Same reason they have a ship psychiatrist. A good bartender is there to listen, maybe over advice. It’s an excuse for a social work point of contact for the person keen enough to know they should leave the replicator in their cabin and maybe socialize. Guinan helped a lot of people. It’s kind of weird that the role didn’t appear to roll up directly under Troi, but perhaps a Civilian social worker was an intentional concession for Starfleet client privilege responsibilities.
@acdw The interesting bit is how much Quark complained about those social work obligations, yet clearly understood them to be a competitive advantage. Even the Ferengi recognize the importance of a good bartender.