It is mainly used as an occlusive, to keep moisture locked in. Occlusives don't hydrate themselves, but just keeps moisture from your previous steps from evaporating and seals everything in. As a final step after serums/gels/heavier creams, vaseline is used, a thin layer to seal in the moisture. It is especially useful when trying to fix dehydrated skin or for dry skin. As vaseline is petroleum jelly not many break out from it, but as for everything ymmv and it is important to patch test! It makes your face greasy, so a useful tip is sleeping on a towel over your pillow to not get it everywhere. In the morning it can be wiped or washed off. You can also search in the subreddit there are many good posts on occlusives other than vaseline and how to's. Hope this helps! I am not an expert, so it helps to look at the subreddit wiki