question #3 Sahih Muslim 700i, Book 6, Hadith 49 : Why is it permissible to perform Witr prayers on a ride but not obligatory prayers according to Islamic teachings?

Salim b. ‘Abdullah reported on the authority of his father that the Messenger of Allah (may peace be. upon him) used to observe Nafl (supererogatory) prayer on his ride no matter in what direction it turned its face, and he observed Witr too on it, but did not observe obligatory prayer on it.