How does leaving leaves on the grass help the bees?
There are so many different kinds of bees, both hive builders and underground dwellers, most of which are beneficial in some way, that this question may have a convoluted set of conflicting answers.
Also do not forget the many ground dwellers who are not actually bees, but wasps or hornets, commonly mistaken for bees due to peoples' ignorance.
For hive builders, the availability of late season, easy to scavenge products for their continued "work" is an advantage.
One point about ground dwelling bees and wasps is that they are all fierce competitors, mostly either carnivorous or omnivorous, this also includes ants, and invade each other's homes in search of eggs and larvae almost constantly.
One way of defeating the enemy's efforts is to make many entrances (and exits) to the underground nesting sites with a few being simply decoys in the case of bees. Preparing for the wintering over for the future of the nest, as they aid the young in pupating, the benefit of close resources during that time is undeniably a treasure.
Not sure how much the purely vegetarian nectar eaters are helped, other than raw materials for nest building and sealing at easy hand.
Yep, I have watched a lot of Discovery Channel and Nature shows. I still wonder how they get those TINY cameras into just right place to get all those shots of the life of these amazing animals.