Cock and balls
The dick produces it to keep the dick head moist. It's a natural lubricant which reduces the friction between the foreskin and the dick head (glans) allowing the foreskin to be pulled back and forth smoothly, and smooth movement between them during sex.
In its fresh wholesome state it's the functional lubricant as nature intended. If it's allowed to build up, however, it changes into an unpleasant bad-smelling creamy substance made up of stale piss, bacteria, yeasts and discarded skin cells. This is the bit we tend to remember!
Excess smegma leads to irritation and soreness which is why you should clean beneath the foreskin (gently but thoroughly) at least once a day.
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"Could these movements be comfortably performed if the surfaces between glans and foreskin were dry and harsh? Of course not. There would be difficulty, delay, and need for manipulation. Nature has therefore provided a 'natural ointment', smegma to ensure easy lubrication and protect this delicate region of the male genital organ." | Joyce Wright, MD | Sexology (New York), vol. 37, no. 2: pg: 50-53. September 1970.