Well, if you ever want to build any real world system other than just a blog or any corporate website (which frankly would better be done with wordpress), then

Well, if you ever want to build any real world system other than just a blog or any corporate website (which frankly would better be done with wordpress), then you're confronted with the fact that everything can't simply be done instantly and / or upon an http request. Think of things like monitoring, reporting, emailing, analytics, cleaning, archiving... the list goes on. For all those things you need a job system. Celery is (to my knowledge) the most popular one.