Not very clear, but petrolatum aka petroleum jelly is used as a barrier so stuff doesn't get into the wound when the skin is compromised. This helps with anti-microbial defense and wound repair.

https://www.jacionline.org/article/S0091-6749(15)01194-X/fulltext

His lips are dry. Vaseline creates a barrier that prevents moisture from evaporating, which improves his dry lip symptoms.

In practice, it is often used to cheaply retain moisture in many dry skin conditions. A doc I once worked with recommended that you put it on psoriasis before bed, wrap it in plastic wrap to prevent it from rubbing off, and bam you're good to go.

Petrolatum is a compound found in sunscreen.

He started working outdoors AKA getting sunburns (if left unprotected, lower lips are one of the prime locations where people can develop squamous cell cancer later on). He should really use sunscreen, which is a barrier (absorbs the UV rays so the skin does not-- rather than deflecting the sun rays.)

Most petroleum jelly today is used as an ingredient in skin lotions and cosmetics, providing various types of skin care and protection by minimizing friction or reducing moisture loss (from wiki)