Bees as polinators
Bees are the most efficient of insect pollinators, and particularly Honey Bees as they don't wonder from flower to flower in the way a Bumble bee does - instead they target a single plant source like your garden's cherry tree, and continue to work it until the nectar and pollen has been taken. It means they are rubbing the flower's pollen on their bodies across the same type of flower, rather than buzzing off to visit an apple tree leaving your cherry gasping for pollination!
Plants produce nectar to lure pollinating insects to visit them, in the hope that while feeding the bees, moths, beetles, wasps etc will get covered in pollen which they'll take to the next flower they visit and leave stuck to the plant's stamens - their reproductive organs that need to be fertilised if they're to fruit and go to seed. There are all sorts of signals that the flowers give to insects to tell them that they have nectar: some change colour (like the the chestnut) when they're fertilised, or give off a small electric pulse that changes when they flower is out of nectar. The plants are all working with the insects to make pollination as efficient and successful as possible.