I'm not in a position to lecture about biology or anything, but some users here could really benefit from just a really basic understanding of some basic biological concepts.
Mark really clearly explained how our body produces endocannabinoids (i.e., cannabinoids that our own body, by design, without any outside influence, makes by itself) to regulate physical and emotional pain sensors. Soubirous had a good post about this earlier. That prefix "endo" refers to something already happening within our bodies, that our bodies are designed to produce and regulate. In this sense at the very least it is simply true that cannabinoids are indeed required for proper maintenance of bodily and emotional health.
As with any other endogenously produced chemical, it can be lawful and in some cases even required to exogenously (i.e., from outside the body) supplement for proper bodily/emotional function. Marijuana is the primary example of an exogenous form of cannabinoids. Using marijuana in a proper amount therefore promotes the required cannabinoidal functioning within the human body.
TLDR: it's required in the same way that vitamin supplements are required. Vitamins are indeed required for properly bodily function, even though not everyone needs to supplement them.