Trees and their leaves smother life underneath them in forests. The leaves do eventually biodegrade and fertilize the soil and in the spring before the leaves grow back the forest floor is full of lots of fast moving, fast living greenery, but then the leaves grow back and usually whats left is stuff that gets sunbeams from breaks in the trees and some hardier species and saplings waiting for their moment to take over. Trees move in slow motion, but they are actually quite active and competitive creatures. Even once their grown their branches smack into each other as the wind blows and they grow competing for sunlight. Some small trees will just give you a little coverage and yeah you can ignore it and it will be broken up and blow away on it's own, but if you have a big tree your hard will get blanketed.

A yard and garden are not a natural forest theyre artifically curated. Letting a yard be smothered by leaves might be good for starting the cycle that slowly turns empty ground into forest in my parts, but most people keep that yard space for activities, or for kids to run around, or for putting a garden in or something.

That said a few leaves wont kill you and you dont have to immediately run for a leaf blower or rake as soon as a single leaf falls on the yard to keep your grass perfect.