Leaves that fall from deciduous trees are a lovely fall ornament to many homeowners, especially those with children who love to play in the piles!
It is definitely okay to allow leaves to hang around for a short while, as they tend to dry out over time., but eventually, it is wise to clear away the remaining build-up of leaves to avoid smothering the turf below, as well as reduced the risk of those dry leaves becoming a fire hazard.
Watch out for certain trees produce whose leaves that are, in fact, toxic to grasses, and must therefore not be allowed any prolonged contact with the turf below.
Oak trees, for example, often display a ring of dead or dying grasses around their trunks due to the acidity (or tannin content) of their leaves.