How did we come by these 39 individual melakhot? The Gemara finds an answer in the biblical text on the severity of the punishment. Which, of course, distinguishes between mezid (intentional) violation and shogeg (unintentional) violation. How this teaches the severity of Shabbat! Also: Gematria! Plus: Isn't lighting a fire the quintessential violation if Shabbat? R. Yosi's position suggests otherwise. That kind of dispute is one of the times where we just pause to recognize the challenges in the Oral Torah, too.