See how important it is to have an oral tradition? It literally helps us understand the Torah/Tanakh. As you know, most of the common laws are hardly described in the Written Torah. Let us take the laws of tithing crops as an example. We must remember that in an agricultural society, such as the society that existed in Yisrael during biblical times, these laws were relevant to almost everyone, every year. Yet the written law leaves much unsaid. In (Vayikra) Leviticus 27:30 we are told that the tithe of the seed of the land and from the fruit of the tree belongs to G-d. Two verse later (Leviticus 27:32), It teaches us that the tithe of the cattle and sheep, all that pass under the staff is sanctified to G-d. (Bamidbar) Numbers 18:21,24 tells us that the L’vi’im (Levites) were granted “all tithes in Yisrael”. Devarim (Deuteronomy )14:22-23 tells the Yisraelites to eat the tithe in the chosen place. In the next paragraph (Deuteronomy 14:28-29) we are told that “At the end of three years you shall bring out all the tithes of your crop in that year and leave it in your gates. Then the L’vi’im can come, for he has no portion or inheritance with you, and the proselyte, the orphan and the widow who are in your gates so they may eat and be satisfied…”
So who gets the tithe? Is it the L’vi’im? Does the farmer eat the tithe in the chosen place? Does it go to the widow and the orphan? This is a law that is relevant to every farmer, it is relevant to a significant portion of his livelihood, and the instructions are not clear at all.
Again, that’s why you need the Oral Torah, it of course answers this.