As they break down, nutrients will return to the soil, although I'm uncertain of how much.
Younger plants that do not yet have a hardy root system will benefit from the 'blanket'. Older trees, hard to say, if the weather was that bad for them, they would be gone already.
We had a lovely maple in our front yard that would turn a brilliant orange, then drop its leaves like I hit a switch.
This, naturally, was at least a weekend AFTER all the other trees dropped, much to the consternation of our next-door neighbor.