Young P.T Barnum from the beginning of the movie and all the other characters from that time period, including Charity's parents, Barnum Snr, the Carlyles and also the Wheelers (Anne and her brother, maybe their parents) all get transported to watch The Greatest Showman.
Other characters from the future may also join them (most probably Barnum's daughters and maybe even future versions of the others or the circus)
This is done to show Charity's parents all that they have missed in their daughter's and grandaughters' lives. Also how much Barnum tried and wanted to do for them and how happy Charity was even with how little they had. It also showed the Carlyles how much Anne and their son love each other and the Wheelers see that their children get a better future. Same for Barnum Snr.