The aravah in the Beit HaMikdash opens some avenues to the kohanim who have some kind of defect, and would be prevented from certain activity in the Beit HaMikdash. Plus, certain leniences of shmittah. Also, the underlying discussion of whether the aravah is a mitzvah from the Torah or a practice from the Nevi'im - a cross-generational dispute. Which leads to a discussion of the levels of halakhah and minhag... Which is then illustrated by cases, as they came before R. Eliezer bar Tzadok, beginning with the aravah, and moving to keeping shemitah down to the most careful details to traveling on Erev Shabbat.