This was the core villain plot of Shadowbringers: Emet's plot was to have you become a lightwarden and rejoin The First to The Source or have you accept you are an Ancient, remember old Amaurot, and switch sides. In his mind he had plotted the perfect outcome because either one was bound to come to pass (in his opinion) and he wins either way. After you defeat Hades, Emet's shock comes when your WOL flashes as an Ancient for a brief moment and he recognizes you as Azem. Since Azem and Emet were friends and he knows that Azem would have sided with the 'lesser races" anyway, he realizes that his plan wasn't full proof, that the WOL was always going to seek a 3rd solution to his plot. And now he was beat at his own game, or his strength, which was plotting and planning.
So to answer your question, he shows you everything in Amaurot so you would take the option to remember your Ancient past. Remember either outcome was a win (in his opinion), plus he was immortal and has all the time in the world to rejoin The First if the WOL would have changed sides.