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 french people go to turky hot spring, the bangcok's naked woman lubricate on french's body and let them slide off with several spa techniques.
Oh. Yehhhh. That's good. UMMMMM. aHHHH.aWWWWWWW.
Bangkok girl lubricate french emmanuelle's body. Each other feel erotic and they gonna fuck after turkey massage.
Lubricant can be used to avoid breaking a condom during vaginal or anal sex. To be compatible with condoms, it should be mentioned on the lubricant, usually water based or silicone based. It may be better than saliva but has to be renewed also. However if you are only looking for casual hookups, it is better that a woman chooses a lubricant that she tried alone first, since some people have allergic reactions (even with organic lubricant) and guys could lose the opportunity to get laid if it happens!
I met a girl who wants to try anal, it will be safer to buy some lubricant.