To make something slippery. A more erotic explanation would be to make the penis or vagina more slippery and hence achieve a better rubbing feeling when the sex organs are rubbing against each other. Men produce a lubricant called precum while women secrete vaginal fluid that makes the vagina walls wet.
To enhance a better sex experience both partners secrete lubricants that lubricate their respective sex organs.
When mom was young she had to wear dresses to school. She said that her Spanish teacher "El Tigre" would look up the girls skirts with a lubricious sneer on his face.
The act in which a couple smears a lubricatedsubstance of their choice onto a 4 foot wire. The female then attempts to wrap the wire around the male's penis with her mouth 10 or more times for amazing pleasure. If the female fails, the whole wire is shoved into her vagina and quickly forced out for quick, jolting pain.