Hugh Jackman starred in The Greatest Showman – so why did Jeremy Jordan sing his parts for the demo? The Greatest Showman demo was used to sell the idea of Jackman's passion project, but he was unable to sing P.T. Barnum's songs, despite being a talented singer himself. The Greatest Showman chronicles Barnum's (Jackman) formation of his "Greatest Show On Earth" which led to the development of Barnum and Bailey's Circus. A musical journey, Barnum gathers performers who are outsiders or would typically be in carnival sideshows of the time, to create a new spectacle of a show, and works to secure funding despite many obstacles being in his way. While not entirely factual, The Greatest Showman is inspired by real events.

https://screenrant.com/jeremy-jordan-greatest-showman-hugh-jackman/