"Je" is and should usually be used in place of a preposition when no other preposition would make grammatical sense. For example, "I believe in God" is translated as "Mi kredas je Dio" as "en" wouldn't make sense since your not literally inside God.
The vast majority of this week was spent looking for this answer. I could not, for the life of me, find one that made sense and explain for what purpose I would ever need "je". Thank you, here is a lingot in thanks.