By pollinating flowers, fruit and vegetables.
Bees are helpful to us by making honey and speading pollenation
BEES
they let bees build hives in them
It depends on what you define as helpful. Yes, all bees have a role in maintaining balance in their ecosystems, especially through pollination, but they are not neccesarily "helpful" to humans directly.
they spread pllen
Nature. Bees are insects. Birds make nests.
they produce honey that we eat
They are neither helpful or harmful. Carpenter bees do not harm humans as they have no stings. They eat and bore holes into wood which may be a problem not any help.
Nature is helpful in many ways, trees give us oxygen to breathe, bees pollinate flowers to give us fruit, swallows stop us getting overrun by flies, its way too much to say in one wikianswer! XD
bees make honey
bees make honey
Models are helpful because they describe nature