Fame
The thing about subs is they're kind of like fillers. People tend not to remember substitute teachers, usually because they don't have them more than a couple times. That is, unless they're really bad. Then they can acquire a certain infamy. Students definitely remember the substitute teacher who tried really hard to teach students how to Dougie and was promptly removed from the campus.
But there's no hall of fame for substitute teachers; no national award for Person Who Most Effectively Occupied Students for One Day. It's a career step rather than a true career that builds a résumé and opens doors in the education field. Unless you become a full-time replacement, even the student newspaper will never, ever have anything to say about you.