If you tell Voorhis that you killed Aryan LaValette, you'll have a different but not-particularly-significant change in dialog with his mother Moly in a later quest. If you wish to encounter Sile de Tansarville later in the main quest, don't tell Voorhis you allowed her to die. If you choose to unlock Letho he'll be available for a quest line later in the game. If you want to be able to access the side-quest Ghosts of the Past and unlock a cameo from Letho of Gullet (antagonist of The Witcher 2), be sure to say that Letho is still alive! Don't steer the conversation away from the topic either, otherwise this content is closed to you. Voorhis will ask Geralt a series of questions pertaining to the events of The Witcher 2, which will affect the world and characters during the rest of the game.

If this option is selected, they will be interviewed by Nilfgaardian general Moorvran Voorhis during the early story mission Imperial Audience.

As such, they are afforded the option to 'Simulate a Witcher 2 Save' when starting a new game. Many players either haven't gotten the chance to play the original Witcher or it's sequel Assassins of Kings.

The long dark save files