You’ve seen how busy bees gather pollen from some flowers — for example, they systematically crawl over the surface of Coneflowers and Black-eyed Susans, the tops of which are dotted with little ray flowers sticking their pollen up for grabs as the bee comes by. Many different kinds of pollinators might walk around picking up pollen from these plants, so there is lots of competition for the pollen. But there is no guarantee that they will deposit any pollen on a nearby neighbor — they might just fly off to a completely different group of flowers instead.
Other flowers present a nectar reward to the bee if it will kindly crawl into the tubular flower, brush against the anthers to receive pollen and then kindly deposit that pollen in another of the same type of flower they subsequently visit. But who knows which flowers the bee might visit next — it might not be the same species at all.
Still other plants produce flowers that protect their pollen for just the right pollinator, one that specializes in picking up pollen from particular a species, and reliably deposits some of that pollen on a neighbor of the same species for some healthy cross fertilization.
By locking the pollen up in a capsule, it is protected from just any random pollinator walking over the flower. Shaking the flower might dislodge some of the pollen, but most pollinators can’t manage that. Instead, bumblebees and some solitary bees grasp the anther capsule with their legs, or even mouthparts, and vibrate their wings at a very high (and audible) frequency — and pollen comes flowing out the pore at the end of the capsule, dusting the bee. This technique is referred to as “buzz pollination”.
The video below illustrates the bumblebee action nicely:
“much of the food we eat owes its existence to the buzz of the bumblebee”