Do the dying/dead leaves that fall off young plants provide any nutrients or any benefit to the existing plant, if left in the soil around the plant?
Leaves are good compost if they are shredded. If left as is on garden beds or lawns they tend to clump and can smother the smaller perennials.
To do my fall clean up I put a bagger on the mower and go at it. I can put up to six inches (12 cm) of fluffy shredded leaves on top of rhubarb and by next June or July the worms have eaten it up. They will do the work of aerating the soil and enriching it for you if they have something to work with.
In the same way we have some native wildflowers who have no problems coming up from underneath 3 - 4 inches (6 - 8 cm). A load of leaves is typical for where they grown and we have had no problems with fungus/virus/bacteria. The plants that worked for us include Meadow Rue (Thalictrum), Bloodroot (Sanguinaria), Trillium, Hepatica.
Although dead leaves have very little nitrogen in them if they are shredded they are an invaluable and free compost. Just don't put it on too thick on delicate plants like primula's.
Leaving dead leaves on top of grass for too long can actually kill your grass. The grass needs the rays from the sun to give it strength and to help it grow. You could take the leaves and put it in some composite pen or composter and take whatever you have in the composter the following year to add to flowers beds, etc.
But leaving it on top of grass and hoping it will help your current plants is a lost cause.
Dead leaves are definitely a good source of nutrients to your plants. You can try and crush or shred them and then mix it with the soil. If you leave it as is, too many leaves may accumulate around the trunk and result in two potential issues (1) the layer of leaves would take a long time to decompost and prevent light and water from going through (2) the moist underneath may attract insects such as rollie pollies that you infest your yard and potentially eat new growth. Overall, leaves are good if you can take some care of your yard.