What was the Sardaukar/Corrino’s original plan with Duncan Idaho?

Spoilers for the first novel and the rest of the series going forward (I’m personally halfway through Heretics but don’t think this question gets answered later).

What was the Corrino/Sardaukar motivation to >!keep Duncan's body and send it to the Tleixau?!< Clearly this became a major point in later books and changed the fate of the galaxy... but not in any way that actually helped the Corrinos/Padishah Emperor tactically or strategically in the original novel. What was the motivation here?