It is very easy to relate with. Everything in life or in nature is designed in such a way that balance must be maintained. If you are the observant type you would notice that this principle cuts across everything in nature. That's why there is rainy season and there is dry season. Imagine if there is no dry season, and It's rainy throughout. It will become pretty difficult for nature to balance itself. In biology, it's called balance in nature.
As for the case study, a standard table is having four legs because that's the only way it can remain balance without it being shaky and unsteady. If the table was designed with five or six leg, there would be imbalance which would make it unable to stand properly, thereby rocking from one end to the other.