Reason #2: His mere presence gives All Elite Wrestling credibility.
For many fans, Jim Ross is THE voice of professional wrestling. He's called major events from the first Clash of the Champions to over a dozen Wrestlemanias. Long time fans can remember him from the 1970's regional era, and NWA fans know him from his days with that company.
For many years, Jim Ross was the voice of the WWE as well. Every Raw, Pay Per View, and many of the backstage or in ring interviews were performed by Jim Ross himself. His ability to remain tonally serious even when dealing with undead cowboys and post apocalyptic warlords was invaluable. His 'normalness' made their strangeness all the more bizarre.
Jim Ross's name has a certain amount of prestige attached to it, and helps to legitimize All Elite Wrestling in the eyes of fans and critics.