He set out southwestward,made his way after great hardships to Palestine,found one friend there and set off again toward the west. (By Philip Khuri Hitti) I couldn't understand why didn't the writer use set out and set off alternatively?
According to the Cambridge Dictionary: there is no difference between these two phrasal verbs:
set off - to start on a trip
set out -to start a journey
I can think of no particular reason why he should use one form in preference to the other. As TeacherKSHuang suggested, he might have alternated their use for variety, but changing just the adverb half of a phrasal verb does not give a lot of variety.
Note that Philip Khuri Hitti was born and raised in Lebanon, although he moved to America in his twenties, where he taught Arabic. It may be that he was trying to express some distinction that is better defined in his mother tongue.