Our Gemara on Amud Beis discusses the derivation and dispute between Rabbi Shimon and the Sages about the walls of the Succah. In one iteration of the discussion, there was an idea that the three verses that mention the word Succah account for three walls of the Succah. Then there is an oral tradition from Sinai about an additional wall - but the dispute between Rabbi Shimon and the Sages was about whether the tradition is to subtract a wall to the number three, hence two walls and a hand breadth, or add a wall to the three, hence three walls and a final hand breadth to make 4. In the conclusion, the Gemara rules that a tradition from Sinai would only come to subtract, not add on.

What is the logic behind the concept that a Halakha Le-Moshe MiSinai would only subtract a wall to the Succah but not add on? The answer to this is connected to an idea we have discussed previously, there is a strong trend within the Oral Torah to be more lenient than the written Torah. The Yismach Moshe (Emor 21) quotes the Arizal as stating that the Written Torah represents Middas Hadin (Divine exacting justice), and the Oral Torah represents Middas HaRachmanus (Divine mercy). Therefore, an oral tradition is more likely to reduce the number of walls required.

To prove this point, I have compiled an extensive list of examples showing how the oral tradition acts as a softening and mitigating factor to the written Torah:

• The literal verses in the Torah famously demand punishment of an Eye for an eye, but the halakha only allows monetary compensation (Bava Kama 83b).
• The scripture mandates 40 lashes for certain violations but actually a maximum of 39 is allowed according to the halakha (Makkos 22).
• The Prohibition of Piggul. A literal reading of the verse invalidates a sacrifice and makes the owner RETROACTIVELY liable for punishment based on the action of someone eating from the sacrifice beyond its correct time (Vayikra 7:18), but the oral law only creates future liability based on past actions (see Rashi Op. Cit). 
• Do not burn a fire in your dwelling on the sabbath (Vayikra 35:3) which implies a prohibition to merely having a fire burning on Shabbos, though the halakha allows a fire to be left on before shabbos to kindle during shabbos. In fact, there is a rabbinic mitzvah to light shabbos candles, which has been a custom of Jewish women for millennia.
• The literal verse states, “You shall cut her hand off” (Devarim 25:12), which is understood as monetary compensation instead of actual corporal retribution (Bava Kamma 28).
• The verse by impurity states, “Or when a person touches any unclean thing—be it the carcass of an unclean beast or the carcass of unclean cattle or the carcass of an unclean creeping thing—and the fact has escaped him, and then, being unclean, he realizes his guilt” (Vayikra 5:2). The actual prohibition is not on passively remaining impure but rather upon entering the Temple or Courtyard while impure (Shavous 16b, and Rashi Vayikra ibid)
• The simple reading of the text in the verse prohibiting carrying sounds as if it prohibits leaving one’s home on Shabbos (Shemos 16:29): “⁦Mark that the LORD has given you the sabbath; therefore He gives you two days’ food on the sixth day. Let everyone remain where he is: let no one leave his place on the seventh day.” The tradition is that this verse is referring to Techum and carrying, not leaving the house (Eruvin 51a).
• The verse in VaYikra (15:33) has a literal connotation that a menstruating woman should be socially distanced. Indeed the Talmud in Shabbos (64b) records an opinion of the “Elders of the earlier generations” who deduced from this verse that a Niddah should not adorn herself or use cosmetics and remain low key.  Yet, through Rabbi Akiva’s position, the Oral Torah interpreted the verse as commenting on a technical aspect; that even if she is no longer bleeding she remains a Niddah until she immerses in the Mikvah. Rabbi Akiva adds, that the opinion of the Elders would cause her to be unattractive to her husband. The upshot is that Rabbi Akiva’s oral tradition mitigates what he saw as an unreasonably harsh standard in the literal verse. (See Ben Yehoyada for even more surprising ideas about Rabbi Akiva and this verse.)
• The literal verse in VaYikra (21:2) enumerates the list of deceased relatives for whom we permit the cohen to become defiled: “⁦except for the flesh relatives that are closest to him: his mother, his father, his son, his daughter, and his brother.” Notably absent here is his wife. The verse introduces the list with a description, “flesh relatives that are close to him”, which in the simple reading is simply defining and introducing the list. However, the Oral Torah derash understands that the “flesh relative” is actually referring to one’s wife (Yevamos 90b). Presumably one’s wife is of his own flesh, as in the verse (Bereishis 2:24) “⁦Hence a man leaves his father and mother and clings to his wife, so that they become one flesh.” Once again, another example of how the Oral Torah provides more leniency than the written, allowing the cohen to defile himself to tend to his wife’s burial.

Occasionally the oral law adds on restrictions but that’s only when interpreting parameters for the overall rule such as defining what is an esrog, wine for Kiddush, requirements to effect marriage, divorce and the like. However, the overall trend is toward mitigating the strictness of the literal word. Why is all this important? It is because as consumers of Torah thought it behooves us to understand its psychological and philosophical underpinnings. 

The duality of the Written and Oral Torah is not a random feature. Rather, it represents two streams of emphasis. The written law is a portrayal of an ideal absolute universal, while the oral law embodies the actual and practical. By way of metaphor, there is an idea called a triangle. A pure triangle, that is one that is mathematically perfect, exists only in concept. As a concept, it is pure truth. However, in reality it cannot exist. The most beautiful and perfect pyramid constructed, while close to mathematically perfect, will never actually be mathematically perfect. The mathematical truth has value in that it is a pure universal. The practical truth has value in that it can actually exist in this world. This is a good way of thinking of the difference between the written Torah and the oral Torah.