Saliva can be used as lubricant, for example a woman can spit on her fingers to lubricate her vagina. It should be renewed more often than a real lubricant since it gets dry faster. Lubricating correctly a pussy can prevent breaking a condom, even if the condom comes lubricated.
If the woman / man is not your official (monogamous) partner, it is safer to use your own saliva to lubricate your pussy or cock that is not protected with a condom, since there is a small risk of STI.
If you are not with your official partner, French kiss / deep kiss with exchange of saliva should be avoided, as well a unprotected blowjob or pussy licking, which also adds a risk of STI.